While all of what you say is true, OP shouldn't be forced into upgrading because of poor DRM support if the game can function fine without it.
It'd be more permissible if the instruction set in question implemented some kind of unique feature that older processors simply couldn't replicate, but SSE is all about optimizing existing operations down into single instructions. Done right, a program should be able to detect what's available and swap in a more (or less) optimized set of function calls where appropriate.
That's the bitch about it though; it's like Microsoft with XP. You have to force people to upgrade or they're just going to sit around with old/unsecured products. Poor implementation or not, Manglement is always going to give us nerds issues with wanting to upgrade the hardware/software for whatever purpose because of whatever reasons. Usually money.
IIUC, SSE also does introduce some security, SSE4.2 added CRC32 checks for instance.
I'm not on the development end of things like yourself; I'm over on the administration side, so forgive me if I misstep some dev stuff on that low of a level.
From the perspective of a programmer and customer on the other hand, I feel that it's a massive cop-out to crank the minimum requirements because the denuvo developers skipped out on due process error checking.
I've heard more stupid reasons in meetings with management on why we're dropping a feature both admins and devs are pushing for than I can remember. I'm pretty sure whatever meeting had both companies sitting down the talk goes something like
Admins/Devs: "This kills the performance of the product and limits the hardware it can run on, it'll piss off customers and they'll just steal it anyways"
Denuvo: "But it protects your product from getting stolen, it's a disservice to your customer to let others steal it easily."
Brass: *Scoff* "WE CAN'T HAVE PEOPLE STEALING OUR STUFF! The sales we'll get from protecting our product will offset the cost of Denuvo and after it's cracked!"
And well, most games that are mainly single player are cracked to hell and back, while your mostly online ones are the ones that take a while to crack. So it really doesn't do shit but make Manglement and investors happy.