Think I'm finally able to articulate why this change to the medical sub rule is annoying me so much.
I actually think having a concussion sub is really important in this day and age, given all the contemporary research that discusses how repeated head knocks affect players for the rest of their lives, not just in their sporting career, in a way that having a reconstructed knee, for example, just doesn't. So I completely understand the desire to disincentivise clubs and players from taking a risk in sending a player back out who's showing symptoms.
The reason the extension to the substitute for any medical reason annoys me is that those incentives simply don't exist with a "conventional" injury. You're clearly not going to send a guy back out who's just torn a hamstring in a close game because it's guaranteed that he'll be useless and he risks making the injury worse. With concussion there was perhaps a tendency to think you can play through it (not the same code, but Sam Burgess in the 2014 NRL grand final springs to mind).
It's blatantly obvious that a player removed from the game for concussion or for an ACL has the same impact on the game in progress - but a concussion rule saves players from themselves (and clubs from liability). Now the message the whole rule is sending is diluted - we're already talking about "how can teams exploit this?" It's a pretty disappointing result.