Only a hots player, but I have watched a bunch of pro league.

someone who worked hard trying to kill an enemy get no reward if they don't get the killing blow

I mean, that's ON PURPOSE. That's basically the whole job of a support in the early game, to never ever ever get the last hit unless no one else is around to take it (and they always should be). Yellowstar isn't making his team weaker by not getting killing blows, he's fulfilling his role.

Watching League reminds me of watching Chess. Extremely fun to do, but you can see when a game is over a mile away. And if you trace back in time, you can see the exact moment when things got out of control. You have the presence of Carries, which LOOK (and even feel) flashy and game deciding. Some get 1v4 kills, ridiculous to even think of coming from only playing HOTS. But a Carry has to get fed. In chess, there are few feelings better than getting an absolute pin. But the setup is what makes the flash possible.

because the killing blow might belong to an ally or even worse a minion/tower

Minion/tower is very bad, your ally might well be the one that you WANT to level up over you, if you want your team to win the game. You can't just take pawns out of the game of chess because they seem boring.

All that being said, they should totally just yank last hitting out of the Big Two, because the internet would melt down and after everyone quit the games their subreddits wouldn't clog /r/all with shitposts all the time. The kids could find a new game to yell at and make dumb memes for, and maybe Riot would make more than one goddamn game in their existence. Maybe Valve could use all their free time to summon Voldemort or the Samsquanch or Gordon Freeman.

If Hots shared some xp with the team and gave the majority to the person who got last hit, it'd make it the game TON more complex and competitive, and also less enjoyable to play. Hots was designed to not have individual rewards for individual performance on your team. Dota and League were designed with a different goal in mind.

League and Dota are fine, or at least people seem to like playing them. Hots is fine, too.

The whole tempest seems to be made by people unable to appreciate that someone else having a cookie doesn't make your cookie any worse. It's like folks are running around yelling at people who enjoy sprinkles on their sugar cookies.

tl;dr - You can make a Zelda game without jumping, but you can't make Mario that way. Choices in game design are not defects.