I have climbed the pyramids in Mexico at mid-day and at midnight (can't do that anymore no matter the time you might pick). Me and my traveling companions were the only ones there. Had a perfect beach in Thailand entirely to myself and my wife for a whole day - not another human in sight. I know people who went to Stonehenge before the fences, and had the place all to themselves (I missed that). I have a good list of special travel experiences like this, too numerous and too personal to share here. These are some of the most magical moments of my life. I'm sure others here could add hundreds of their own examples.
Travel has changed. The world has changed. Billions of people have found their way into expanding middle classes in places from China to India to Mexico and lots of other places where only a generation ago, international travel was a dream just available to a tiny wealthy elite. Millions who previously couldn't now want to - and can - see the Mona Lisa, Venice, and a hundred other places you want to see. Can't really hold that against them (even if you might feel, like I do, that crowds ruin the experience).
If you want amazing places uncrowded like they were 10, 20 or 50 years ago, you either need a time machine, or you need to find some other way to be OK with sharing them with thousands of strangers. Or, go someplace that mass tourism hasn't yet discovered (and perhaps for some, ruined). That's not easy - tourism has become a giant, worldwide industry. Finding someplace where there aren't a hundred (or a thousand) people waving selfie-sticks is not easy. It generally takes some effort, lots of time to get there, some inconvenience, maybe some discomfort, in many cases a bunch of money, and an investment of your time to figure out where to go. It's still possible. Not easy, but possible. Places that are famous, easy to get to, inexpensive, and make instantly recognizable backgrounds for photos posted to social media - they're going to be overrun.
The lesson I have drawn from watching things change so much over the years: get off your duff and go as soon as you can (wherever it is you want to go). Don't put it off, go NOW. You wait 5, 10, 20 years and it's not going to be as good as it is today. Just my opinion, from someone who hates crowds. Places with no cruise ships, no tour buses, no crowds with selfie-sticks...they do still exist. Go and get there quick (but not until after I blow through there, please).