That effect for 4 mana is absolutely bonkers. You can basically always draw your entire deck if you untap with this/have enough mana. It's basically a permanent Mind's Desire.
By far the main thing holding this back is going to be the colored requirements. It's easier to generate 3 mana of the same color than 3 specific and distinct colors with things like black lotus.
You probably can't play this in a control deck since you can't really use this incrementally and you'll have a hard time closing games in one turn.
So, you are left with a combo deck. You need a shell that can play lots of spells to make sure you make the most of this card and an on the spot way to win. So Mentor, Tinker, Storm kill and/or Time vault.
I just don't see the Doomsday/Ritual deck that can support this and Gush being restricted means no Gush storm either.
So, you are probably looking a lot like an Outcome deck at this point. Are you playing this over Outcome or are you playing both? and if both, what are you cutting for this?
Unless you can somehow put together a Baral shell around this with manamorphose? How good would this card have to be to make a deck like that competitive?
Prices for vintage cards are currently going down, and are likely to continue doing so in the near to mid future, so I would wait until the prices bottom out before buying into the format.
Another factor you might want to consider before taking the plunge is that the paper vintage scene is nowhere near as dynamic as Vintage on MTGO, you'll be lucky if you get to sleeve up paper vintage once a month, depending on where you live.
Are you willing to invest a significant amount of money into a hobby you'll only get to enjoy once or twice a month in small events and only once or twice a year in "high-profile" events?
As far as Dredge, there will always be a Bazaar of Baghdad deck in the format and even if something obsoletes dredge, it will be a better bazaar deck (like dragon was the BoB deck of choice before the dredge mechanic).
No amount of WOTC printing backbone breaking hate has been able to push Dredge completely out of the meta so far (see Ravenous Trap, Leyline, Rest in Peace, Containment Priest, Grafdigger's Cage... etc). Even when it is not the best-positioned deck and is being gunned for, it is still incredibly resilient and has inherent strengths against many archetypes.
It has existed for over 10 years and has been forcing vintage players to pack 6-8 graveyard hate in their sideboards to have a fighting chance ever since.
In fact, it just got a retooling with MH which even led to it getting its first card restricted due to unprecedented metagame shares, so it's about as good as it's ever been right now.
There is also already another competitive bazaar archetype in Survival, so even if dredge becomes horrendous or if you want to play something different you always have that option.
I guess this sort of fills the gap that the restriction of misstep opened. It gives a serious tool for tempo and control decks to push decks like PO and DPS into the mid-game where they can start piling up more hate pieces and/or get their draw engine + counter wall online.
Seems maindeck-able in Hatebears and Landstill, greatly increases consistency of turn 1 plays for Hatebears and helps Landstill by pushing the opponent into its long game gameplan and preventing more "veloce" blue decks from overpowering it with multiple broken spells.
This also seems like it could be really insane in a Bazaar deck (at least theoretically), since the tempo generated by this card is enough for a Bazaar deck to beat any deck that could potentially race it otherwise.
As far as Critical mass, I am referring to the Critical mass of available spell combo enablers: Memory Jar, Bolas's Citadel, Necropotence, Yawgmoth's Bargain, Oath of Druids, Gifts Ungiven, Paradoxical Outcome, Dark Petition, Burning Wish, Wheel of fortune, Timetwister, Tinker, Windfall, Yawgmoth's Will, Mind's Desire, ...
Draw7s are effectively almost as artifact mana reliant as Paradoxical outcome if you want to cast them in the first two turns and/or be able to consistently win the game after resolving one. If neither of those are part of your goals, then that begs to ask the question "why would you want to play those cards in the first place?".
But, doesn't the fact that you have to set it up make it worse than a card like timetwister?
I am not referring to the riddlesmith list, which I think completely offsets the drawback of the card and actually benefits from it. However, it remains to be seen whether that deck can outperform current combo decks.
I was thinking more along the lines of our established spell combo decks like DPS and PO, where this card is rarely going to be good and which would render those decks worse overall if you had to find room to add this card and a discard outlet.
At prima facie, this card does not belong in a Bazaar deck or a deck that would play Dack Fayden, which are the most obvious discard outlets in the format. So you would have to find another outlet which has synergies with a deck that would want access to the draw 7 effect. Riddlesmith seems like a good fit here, but I have not played with or against it.
I'm basically bringing up some of the arguments found in this article.
Given that it isn't good enough to build around in legacy, where the available combo enablers are much worse and where you have access to 4LED (which pairs divinely with this card), I have a hard time seeing this as relevant enough in vintage, objectively.
I will definitely acknowledge that this card has some very strong and unique qualities (First card of it's kind which makes it hard to assess, opens design space, increases threat density by having the potential to be a 2 for 1 against counterspells, the effect is very powerful, doubles as disruption against Bazaar decks)
But in the end, Echo of Eons seems too constraining for me to envision as viable (at least theoretically) and seems to suffer from the same issues that a card like Day's Undoing does (albeit less restrictively so).
I do hope I am wrong though because this card is definitely sweet and fun. I have full faith that if this card is breakable, Chubbyrain will find a way to build the deck that does so.
Why would you want to play this card over the plethora of alternatives that are all more broken and more practical?
What does this card enable that you couldn't do before that is better than what the current combo decks are doing?
I have a hard time buying that you end up ahead vs a control deck by putting 6 mana into this card (which can be pyroblasted or flusterstormed) to get it countered and recasting it on the following turn. Or even that this is a frequent enough occurrence to be reliable.
Another thing this does is that even when not big, it still functions as a removal spell or a tempo play.
I could see this in the sideboard of a combo deck where it either provides an alternate win-con in the face of hate or it buys you enough time to stitch together a win.
It's hard to evaluate how reliable this card is though, which will ultimately be the deciding factor on whether this is actually playable.