Most of us here have already travelled to Europe. Some many times and some even live there full or part time.
We generally make one or two trips to Europe each year, and often a mix of larger cities and smaller ones or the countryside.
I have yet to return from a trip and say, "I'm done with Europe!" over tourists. I have come back saying, "I should have known better than to visit during time. Next time, I gotta remember to think that through."
A recent example would be in Lisbon and making the RS recommended visit to Sintra. That place was like a hilly disorganized Disneyland, but that was as much our fault (day trip and slightly late start) as Sintra's. While I liked the Moorish castle ruins and wandering up and down between the sites, the crowds were relatively large, and the lines for places like the Pena palace were long (timed entry) and ill-defined/unregulated. To me, the easy complaint would be "over-tourism" caused that headache in Sintra, but digging into it, much more of the problem was the logistics and infrastructure of the location. If the tourism folks there worked out a bunch of the weak spots - like addressing the messy Pena palace lines, maybe reducing entry tickets, and other tweaks - it would vastly improve things. I personally wouldn't recommend copying what we did for Sintra, but with some mods to how we did it, plus Sintra upping their game, it would be a future recommendation.
But having been to most of the current "hot spots" for over-tourism stories - like Barcelona, Venice, Amsterdam, Santorini - I can't say I've ever been flummoxed by my overall experiences there. Barcelona's Rambla? CROWDED. Venice's Piazza San Marco? CROWDED. Amsterdam's Red Light District? CROWDED. Santorini's Fira near the cable car? CROWDED. All to be expected in peak travel season. But we've never not simply been able to adjust our visit by heading away from those areas. Venice has a huge portion of the city that is NOT like that and can be avoided and greatly enjoyed by heading away from the crowds. Same for most/all of the "usual suspects" for over-tourism complaints.
There are absolutely places that - to me - feel too crowded, and that's where I adapt and find the options more to my liking and comfort. For me, it's hard to ruin a trip due to crowds UNLESS I chose to dive into a crowded destination and opted to stick to my guns and not tweak my itinerary. That hasn't happened yet, but occasionally, I am reminded to stick to our "normal" rules of not traveling somewhere in the height of season if we can avoid it or the destination offers easy non-crowded options. And, usually, there are plenty of non-crowded options in almost any destination. Just don't plan on getting the Mona Lisa or the Trevi Fountain to yourself in July. :D But that's on you if you planned the trip thinking that was happening and then feel it is "ruined" by tourists and crowding.