Rockstar Games is in a really unique position. For one they popularized open world games with GTA 3 so they were basically "first." Being there first with a gigantic new genre is safe to say a great advantage, just look at Fortnite.
Being first has ton of advantages, they've built up a reputation not only with gamers but also with critics which means they can land 10s easier, a huge advantage in a competitive market.
The driving in GTA 4 and 5 wasn't fun at all, the guns sound mediocre, despite being open world their game design is really restrictive.
And perhaps most importantly gyro controls is a necessity for any shooting game, it should be industry standard, autoaim shouldn't exist because it'd ruin not only the single player but the online, too.
There's no reason why Splatoon should have superior shooting than a billion dollar game.
If GTA 6 comes out with autoaim/no gyro etc and still gets 10s that's the being there first edge, I don't think any other developer would get rewarded for worse gameplay.
I'm sure critics will talk about some new feature like horse balls again like that's an excuse the core game mechanics aren't any fun.
But I don't think there's any question custom engines are the way to go. I think any gamer that celebrates when their favorite franchise switches to Unreal Engine are simply ignorant.
The reason why Unreal Engine is gaining popularity is because the gaming industry is scalper based, most companies fire employees for any reason they want (just look at Microsoft deleting Tango Gameworks after they said they wanted "smaller games that give us prestige and awards" which exactly what that studio did) or employees leave because they have to work unreasonable hours and then new employees have to learn a custom engine which takes time which no company wants to waste.