I assume you're saying that because your proposal isn't that different from what they do now.
Of course, that also makes the converse true: Since what they do now isn't that different from your proposal, that means what they're doing now is likewise reasonable by your standards.
Note also this isn't free for them; it costs them money to maintain security support. Plus Apple themselves have said the security support they offer during the two year extension isn't as complete as what's offered on newer OS's. And I suspect it would get more incomplete as the computer gets older.
Hence Apple may not want to offer that extra year or two of security support, if it's not up to their standards, since they don't want this to mislead customers into thinking their machine is decently-protected when it's not.
Again, not saying it wouldn't be nice to have them extend it to a decade. But I don't know how much of their decision not to do so is purely economic for Apple, and how much if it is practical. Probably a combination of the two.
Come now, "paperweight" is hyperbole.
Older machines can continue to be perfectly functional. In addition to my 2019 i9 iMac and 2021 16" M1 MacBook Pro, I also have a 15" 2014 MacBook Pro that I use frequently to sync my iPod and also when I want to go mobile and would prefer something lighter than the 16" M1.