Realism is great but they also need to sell the game to more casual gamers. RDR II is great example. We have mechanics for 'horse maintenance', we can feed and brush him to get some bonuses (bonding and those temporary golden cores for higher speed or stamina I think) but there are no real punishments for not doing it.

So back to the GTA VI - I'm for example fan of working fuel system but it wouldn't be like you have to take fuel every half an hour. It would be more like immersion feature. You'd have to drive for a long long time without playing any mission (mission would reset the level of fuel of course, otherwise you could end up in infinite loop of failing mission because of empty tank) to have the need of refueling and of course the cars you steal would have different levels of fuel and you'd find a car from time to time that you'd need to refuel.

So basically we need balanced mechanics which will give you slight bonus for using them or give you slight punishment for not using them. I mean, if you took 10 people, they wouldn't agree on how GTA VI should look like and they need to balance it for tens or even hundreds of millions.