Yeah, Parker started out as a teenager, but he hasn't been one for decades. I mentioned this in my post, but I like to see the comics reflected as much as is reasonable, and Parker hasnt been a teenager since forever ago. That's not reflecting the current source, that's just touching upon the basic origin. It's sort of like making a movie where Batman is a gun-totting psycho who kills people. That's reflecting the original source, but its so old no one actually remembers that or thinks of Batman in that way. A younger Parker in the films while he's thirty-something in the comics wouldnt be a deal breaker for me or anything, I'd just personally like to see things line up better than that.
As for Infinity War, you're likely right. Most of the current actors are looking at their end dates and will have to be replaced in one form or another. But since I have no idea what Marvel will do when they get there (in what, another five years?) it didnt seem overly relevant. Maybe Marvel will go in that direction or maybe they'll just re-cast the same characters (like War Machine was). Who knows?
As for the content, or lack thereof with Miles, yeah its a thing, but its minor. Most Marvel films barely follow any kind of established story, and Miles has enough of a core supporting cast of his own to work with. As for the villains, it doesnt matter. None of them are well known among the masses, its not like audiences are going to say "Hey, The Vulture is a Parker villain! How dare he show up fighting Miles!" Audiences wont know, and they wont care.
Again, this is just me. And as Ive said, Im not the biggest Spidey fan out there. I just figure, I've seen five Spider-Man movies in fifteen years featuring Peter Parker. And while there's tons of stuff yet to be done with him, Im over it and ready to see something different. And ultimately, its moot. Marvel is going with Parker, and Im sure the films will be amazing and I'll enjoy the hell out of them. Im just saying, I would have liked to see them be a little more daring and go with someone else.
Or, they could be bats**t crazy daring, forego Parker completely, and just roll with Spider-Gwen!