Riot's view is that you shouldn't leave games, and making it easier on your team after you leave doesn't stop people from leaving. Thus, their solution is punishments for leavers. Why? Because long term, it will solve the problem, once everyone who habitually leaves is removed from the playerbase. Whereas if we have replacements, people will continue to leave, and leave even more cause they know their team won't suffer for it.
Should people be allowed to leave? That's a value judgement, and people feel differently about it. Does real life trump games always, or do you have a commitment to your teammates to stay for however long it takes, winning or losing? Somewhere in between, IMO, but there are people who will leave for pretty much any trivial reason(to get a snack, to go to the bathroom(cause they didn't realize they needed to go before the game started...), to go watch their second favorite TV show, etc.), and there are people who have to be dragged out of burning buildings.
I think what you're suggesting will increase the number of leavers, but decrease the overall penalty they have on games. Sure, it sucks to have your 5/0/7 Jinx replaced by a first time player, but it sucks more to have them replaced by a vegetable. But Riot's already made their position here, and I don't think they're going to change it. And, as a player who will play through most non-emergency situations, I'm sort of looking forward to the day where there are no more people leaving to eat dinner cause they didn't plan ahead. Sort of sucks in the meantime, but I think it'll get better eventually.
TL;DR Riot hates leavers, so they're going to solve the problem by banning them or forcing them to improve their habits, not by making games with leavers better.