Mistwell
Crusty Old Meatwad
There's a very strong tension between the first line and the bolded bit, here. Unless your campaign provides roughly equal weight to combat, exploration, and social encounters, you're making it hard for the first bit to be true. Further, it ignores the very real issue of people "turning off" when their PCs are rules-excluded from a scene.
First, this assumption that anyone is ever "rules-excluded" is false. And it's the heart of my issue with this. NOBODY is arguing that characters will be rules-excluded from anything. And as far as I know, nobody has ever made that argument. But this is not the first time I've seen that claim made by people who care a lot about balance.
It's a strawman, and a common one. In my experience, when a strawman is used this often, it's a sign of an underlying problem with the position being put forth - something that's not being said, or not being examined.
If your players are "turning off" when their PCs are not shining, then you have a problem with your players and probably your DM as well. That's likely the issue not being examined enough in that situation, not the rules. If people are routinely "turning-off" like that, then I doubt any amount of balance will really address the issue you have at hand. Simple luck means someone will often be shining while others will be ineffectual just because of bad rolling or good rolling, sometimes for long stretches of a campaign just by coincidence. That game is destined for implosion if you have players who think like that, and DMs who don't know how to address it. And masking those problems with the rules (maybe damage on a miss, or DM dice fudging, or something similar) might help for a while to smooth out those rough patches, but ultimately I think those games end up having really bad problems unless that underlying issue gets addressed.
Another reason acting is a crap comparison is that actors know what they are getting in to. If you agree to play X part and understudy Y, you know the minimum and maximum amount of what you're going to get to do. If you're told you're only walk-on extra #112, you know it. Whereas a PC can be excluded by a careless DM, an adventure which excludes his role (it may even be a good adventure, otherwise!), or simply dynamics, and has no idea beforehand that this is going to happen.
I don't agree. Sometimes, in fact often, an unexpected role steals a show (in fact that's why that phrase "steals the show" is so common - one unlikely role overshadows the lead). Sometimes Beatrice is the one who shines in Much Ado About Nothing. Sometimes it's the performance of The Nurse that audiences remember from Romeo and Juliet. It's taking what you're given and doing your best to make it shine that's the challenge.
And sometimes it's the low ability score in role playing, or the unexpected effectiveness of the bard in combat against the odds due to the player's cleverness in using their environment or forgotten equipment, that ends up shining.
I also think not all games are meant for all people. If you really enjoy balance, if that's one of the highest things on your hierarchy of what makes for an enjoyable game for you, then I think perhaps 4e is a better choice of game for you. Balance is a strength of 4e. I think some balance has been (necessarily) sacrificed in 5e to better some other aspects of the game.
For me, balance isn't as important as some other aspects of the game like flexibility, reduced quantity of rules, stronger niche protection, and faster combat. And given my preferences, that likely means 5e is for me.