Stop being stubborn man.
A 720p monitor showing a 720p picture will be natural size when using full screen.
If you use the same 720p monitor to show a 4k picture it will crush everything to downsample so you will lose to much detail that you may not see a diference against that same other 720p image, but if you allow you to show a 1:1 (or as you insist in call zoom in) then you'll see a lot of difference.
But if you prefer to look at a better comparison, pick up a 4k monitor and look both pictures, the 4k image will fill the whole screen, the 720p will only cover 1/9 of the screen (then yes if you ask it to do full screen you'll have a zoom in) and it will look terrible, but you wouldn't game using 1/9 of the screen so that how a 720p image shows on a 4k screen.
And of course you haven't really seem a big leap in 4k screens in store, most of the time they aren't even showing 4k content.
You are basically doing confirmation bias, putting yourself in situations that you can't see a difference and saying the ones that you can see the difference are wrong, when it is actually the opposite. The ones you see the difference are the right way to compare.
Making it so that the middle 1/9th of the screen fills my monitor only tells me what the difference would be if I either had a huge screen or put my face right up close to it.
I'm perfectly content with sitting 3 meters back from a 47" 1080p screen, and much like the 720p vs 1080p debates of 2013, I honestly just find the whole 4K thing to be overblown. That's just my personal opinion.
Bet with Liquidlaser: I say PS5 and Xbox Series will sell more than 56 million combined by the end of 2023.