It is no different to balance compared to now where there being level 1 characters and level 25 characters fighting the same monsters as it is now. Or if you are 4 level 5 friends who are playing together and fighting a monster.
I am a level 10 character now playing in the level 5 area. So by your logic other people who are playing by my side are getting experience for my "legwork" in the same way when I run around killing enemies with one blow that take them 5 blows. It is the same thing. That is actually worse then the reverse, there would be less instances of powerful player coming in and "stealing" your kills. And you would not get kills easier just because there are some low level player fighting the same monster as you. Remember that with the scaling you are both dealing aprox the same amount of damage.
STO had a difficulty slider. You had instanced quests and you could choose normal ,hard etc.
It worked pretty well but the quests were instanced. You could even use a group finder that would pair you up with people that had the same difficulty setting.