The only answer as far as I can tell is increased taxes and costs to visit sites.
National Parks are implementing reservations to prevent mob scenes in certain areas. Nobody wants to visit "nature" and be surrounded by hordes of people.
I think you will see more cities, particularly mid-size to smaller cities where tourist influx is a real burden, adopt more "Pay to Play" measures.
If you read lots of articles on this topic, what you find is that no one is saying they want no tourists, or for the most part even "fewer" tourists, they just want the ones that spend lots of money. So the focus will be on limiting and charging a premium to see prime sights, strict controls and taxes on Hotel and let room rates. Also, while many laud the cities for going after the iconic "Drunk British Package Tourist" and Stag parties as well as those terrible cruise ships, they have no love for the day tripping budget traveler either.
So yes, I think one needs to look for more "back doors" in and out of Europe, however, for myself, and many others on here, we have that luxury, we have seen Paris, London, Barcelona, Madrid, Rome, Amsterdam Venice, and more, multiple times even, so visiting places well down on the tourism list is just natural, maybe needed. But, Venice and Amsterdam, and...are all popular for a reason. If I were just starting out on my travel journeys, you bet that is where I would be heading.