And someone who likes to roleplay and not do all the quests would also like to reach the max level and will be a bit disappointed if the game is too challenging for him. Or some players just won't bother doing them all. The developers have intended from the start that people who do all the content will reach max level before the end.
This happened in all the IE games. In BG2 if you did all the quests in Amn and the stronghold ones you would breeze trough the rest of the game till almost at the end because some quests should have been done in Act 5 when you return to Amn. Of course certain quests were a bit hard, especially Kangaxx or the Twisted Rune but even without cheesing the game it was still doable with some strategies and luck
If the game is too easy play with a smaller party. You can just rotate companion to do their quests s you can see everything.
If the XP is changed to fit most solutions here it would force everyone to do everything.
Some people are completionists, myself included. Honestly I redid a small part of the game three times to do the quests of all factions. I understand how it is, to have a need to do everything. But not everyone is like that. Again some people roleplaying won't do some quests for a large number of roleplaying related reasons. To prevent roleplaying in an Role Playing Game is just dumb.
Current system makes everyone happy, just that completionists need to employ certain self imposed challenges some of which range from minor ,like playing with a party of 5 instead of 6, to major, like not leveling up, if the game isn't challenging enough even on Path of the Dammed. Why put everyone on a disadvantage forcing them to play with a full party even on lower difficulties just because they need to do 95% content of the game to reach the max level.
What if they don't like other party members? What if they like to play with a smaller party?