There are a few things to be noted there.
First, the number of people "discovering" that they might or need to buy reservations in advance is increasing, but it doesn't mean the number of visitors to those sites has risen in the same proportion. It is just a general change in the way people travel, the same way trains 40 years ago were mostly not subject to reservation and relying on fixed prices published on a written annual catalog, and now they are sold like airline seats. Gosh, even air fares used to have "banded pricing" and in some routes actually "seasonally fixed pricing" defined by bi-lateral agreements between authorities in both countries, before we got the blessings of a more free market.
What I foresee is more and more sites adopting compulsory pre-booking. I actually long and wish for this development: that all major museums and other confined spaces adopt advanced ticket purchase as a condition of visit, so I can enjoy them with greater peace and less crowds.
Some people, of course, will claim this will further kill "spontaneity of choosing your daily schedule as you wake up and take a look at the window".
As for alternative destinations... good luck on that. I agree with the idea, but even here on this forum, where people are supposedly open-minded about looking beyond the obvious, it is extremely common to read seasoned members shut down people who are going first time to Italy and are considering some cities beyond the famous ones. Only the "anointed" back-doors are legit, the rest is "messing up with the right priorities". As in "Bologna might be nice, but on a first trip to Italy you shouldn't skip Venezia for it".
Even at the city level this happens. I've been in Venezia couple times, even on the busiest days, barring flood, you can easily find quieter spots that are just some 5 min walking out of the 3 or 4 major walking routes. It is really not difficult at all if you have a smartphone/tablet with an offline map to wander around most tourist crowds, even in the first Sunday in August. It is also fairly easy to beat crowds in Amsterdam - it doesn't take much to have pleasant walking routes that are not a concern for crowd control safety officers. It is just that the insecurities of missing "something" on these major constrained routes in major cities is too much to bear for many people, I think.
One of the greatest benefits of summer traveling are very long days, I don't understand why many people don't take full advantage of that, especially regarding open places with free access that will be under sunlight well past 8PM.
Finally, cruise ship crowds are a separate issue on their own. Ports in places like Venezia and Dubrovnik should ramp up mooring charges so that this money can be used to expand infrastructure for "peak cruise tourism" (starts at 11AM, ends at 4PM, leaves little to no money on the shops in town or hotels).