Here's an alternative line of thinking that should eliminate all your issues. I know it won't because you sound like you're angry that people don't play the way you want them to and so you'd like to stop them from doing so. But the premise of your argument is that these Great Buildings shouldn't be allowed because they are in the player's future eras. The reasoning is sound if you consider players to be in a bubble where building things is concerned.
However, I don't believe the tech tree is something that the people of your town "discover" as they get more advanced. I believe the tech tree is something that your people become more self-capable with. Meaning... just because you can't produce a higher-age thing doesn't mean that this higher-age thing is unknown to them or impossible to acquire. You have friends. You have a guild. Those folks are capable of "investing" in your town. In the real world, this happens. A lower-resource, less capable society may not be able to develop high-tech industry on their own, but that doesn't mean the infrastructure can't be built by someone from outside their community... even using labor from within the community to do so. The tech can't be built locally because they don't have the means, but they still have access to it. They might even look strange with a cell phone tower in the middle of a mostly farm-based community. But if cell phones allow them to export their goods and bring in much-needed trade, then who are we to look at them funny? Maybe as they grow and thrive, they will advance and be able to do more and more internally. But in the meantime, they shouldn't be relegated to sticking only to the things they can do.
So... a town that is centered around the appeal of a low-tech kind of "ancient" tourism shouldn't be expected to remain in the age of that tourist draw for all things. For one, it wouldn't be a very good tourist attraction if it did. You still need ATMs, cell phone and internet access, etc. for the tourists and for the people in charge of marketing the tourism, among other needs. You may be in a lower age of tech internally because your town is "mostly" stuck in the middle ages, for example, but you've added some key components to allow your middle-age town to thrive as a tourist destination. Perhaps a modern hotel for folks who don't want to sleep in a barn. Maybe a bank so they can spend money in your town and you don't have to keep that money under your mattress. How about a nice restaurant for people with food allergies so they know you aren't going to kill them for dinner. These are examples of towns that might be internally low on the tech tree, but that have had things introduced as amenities despite the low-tech nature of their existence.
The thing about camping in a low era that folks dismiss is that it doesn't necessarily have to be thought of as a town stagnating at a low level, but could be thought of as a town that is purposely not "aging" itself, but still finding ways to advance that are more subtle, but highly useful. Many European towns are visually stuck in the past, but that doesn't mean they haven't found a way to generate electricity or offer indoor plumbing. If we age our town using the tech tree, we lose our "quaint old town" appeal. Many towns in the real world purposely hold on to this. I see no reason why we shouldn't be allowed to do the same, while still finding ways to exist in a modern world as well.