Not to say you can’t be bothered, but I look at it this way. I’m honestly only really looking at a small amount of my screen at a time…. I suspect I use that space very little.
Maybe you feel that way, but it's not a commonly shared belief. There is a marginal value to having additional screen space, and its silly to block use of it via software gates.
So, in the end you're saying screen real estate is important? Or not? You have to pick one to be logically consistent.
As far as the value of larger versus smaller screens, I can also relate to OP. When I’m sitting with my laptop on my actual lap, I definitely have fewer things on screen at any given time than when I’m using my comically large 43” desk monitor. I’m generally not looking at more things on that monstrosity at any given time! But being able to “switch” to them without interacting with a mouse/keyboard is a convenience that the larger screen affords. Even if I don’t care about trying to shove more of my active focus into the top few pixels of either screen.
"More space matters, but not if it's 100px at the top of the screen"
If you are a believer that additional screen space provides some marginal value, as the majority of people do, then you cannot at all argue that there's nothing lost by losing screen space. If you believe that you only need 15" and anything beyond that does not provide additional marginal value, then you can be logically consistent with these two statements.
If you don't see the logical inconsistency, I can't help you, sorry.
You absolutely cannot argue in good faith otherwise. Its a provable mathematical fact that the beliefs are incongruent.
You are free to choose to hold logically inconsistent beliefs though. You do you