Ugh...so many problems.
The Karn PO deck was partly developed on MTGO, with contributions from JP Kohler, Jeremy Beaver, and myself (all of us have MTGO accounts). I tested exclusively on MTGO but I can't speak for everyone. I streamed the Mardu Planeswalker deck multiple times, even spoke with the creator about an updated list. These lists often don't persist in the more competitive MTGO metagame but that isn't for lack of trying. Stax sees play and had a 5-0 recently. I haven't tried Sean's list as we have different deckbuilding and format philosophies, but that doesn't mean others haven't tried it on MTGO.
There is a Dragon player by the name or Redlose who has several 5-0s with it. And if you check this week's lists, you will see Cindervines Jund, Dark Bant Hatebears, Turbo Depths, Brian Kelly's pile, along with the usual suspects. To say that none of these decks ever show up on MTGO is inaccurate and honestly offensive as I have made it a personal mission to do just that.
As far as you playing the same things over and over again, do you have data to support this? I kept track of my league matches since Lavinia was printed and the only archetype I have encountered over 20% of the time has been Xerox. Every other archetype is around 5-10%. You play these decks once every other league and xerox every league on average. Now, it may not feel like it. Selection bias occurs especially when you are unhappy with a deck or play pattern in a format . You remember the games in which you face triple Hollow One off a Bazaar but those games aren't representative.
Regarding unpowered decks, yeah, they are bad. There is no competitive advantage from limiting your access to the most powerful cards in a format, which is why the TOs allow proxies or give additional prizes. And it's why these decks don't get played on MTGO. Unpowered decks are not a feature of the metagame, but a bug of the high cost of the format. Specifically, the cards that you mentioned as enabling unpowered decks do so because they are powerful themselves and proved to be oppressive in powered decks. Chalice was able to ruin 20% of Vintage games in conjugation with the die roll, and would be downright absurd in modern Ravager Shops.