Cyberpunk had the right idea making the main quest shorter and expanding on the side quest.
I do think open world games need a big fundamental design change.
I just think in these 2nd or 3rd games you can get away with saving the "open world" for last because most of the time you end up spending way too long on the side stuff and you level up and get so much stronger than most of the main story enemies end up being a piece of cake or not the 1-2 shot kill danger they are supposed to be.
Locking in on mostly main story missions keeps you invested on the main plot and it's priorities. By opening up the game completely only after you finish the main story you also open up a lot of possibilities: side quests that are a consequence of what you did to beat the main quest, character that talk to you with all the knowledge of what've done, people getting pissed at you, new enemies showing up, bad guys getting a hold of weapons/technology/lands/vehicles and wanting to take shyt over, etc.
You also don't need to start over every game. Let me keep the cools shyt I learned on the first game or most of it. That way you take out the necessity of leveling up so much and do side shyt to get powers/weapons/exp/ that you need in order to keep advancing the plot.
Imagine being able to do something like Last of Us 2 and after the final fight you get to actually live in the world you've been traversing and face completely new challenges, some of which are a direct consequence of what you did to beat the game? It would be a cool way of spending the 30-50-70 more hours you do in side stuff in open world games anyway.