@MaximumCDawg In principle... I think you are absolutely right.
It is kinda interesting to think though, about this thing being a creature. That lowers its effective casting cost against Shops a lot. Again, 5 of the 9 taxing cards they run have no effect on this... making it, in terms of probability, much easier to cast than even a Crucible.
Math: 31% of top8 decks run Thorn, and average 4 per deck. 28% run Sphere and run 3.9. 26% run Trinisphere. (Also 9.7 run little Thalia, at a 3.9 average)
So if your opponent draws a hand of 7 and stares at you, there is a 12.3% chance they're holding Thorn, 10.9% they have a Sphere, around 3% they have Trinesphere, and about 4% they're holding Thalia... also, a 0.00002% chance that the guy is some hapless standard player who is about to watch his dinky Mardu Vehicles get stomped off the planet by magic as St. Garfield intended it to be.
Getting all those taxing effects down to one number is tough for a couple reasons, but without getting into it too deep, I'd guess its something in the neighborhood of 23 or 24% that they are holding some taxing effect. (This is guestimation though, and can't really be derived with precision without sum'a uncle ray's down home big data scrappin'... but its close anyway.)
SO let's say that barring the Mardu Vehicles scenario, there's about a 1/4 chance you are facing some taxing effect... now consider that if they are holding a taxing effect, the only one that leans on Excavator directly is the Sphere... 10% they are holding it... and even if we know they are holding some taxing effect for certain, there is still only 74% chance that card is not Sphere, and thus has no direct effect on our casting the Excavator