Only if the PCs actions warrant it (what if they're really clever about not using their magic in obvious ways) or the GM forces the issue.
You seem to be assuming D&D-style fireballs, but that's a big assumption. Look at Mage: the Ascension, where you're required to make your magic as unobtrusive as possible. If you want to shoot a lightning bolt at a target, that's bad; it causes Paradox. If you want to subtly rearrange reality so that a car crashes into a utility pole and causes the power lines to snap and one of them to hit your target, thus electrocuting them, that's fine, because it seems to be a mundane event.
And in D&D, most spellcasters are going to have access to a very large number of spells that do all manner of things. That's not true in other games; in many games, you only get a handful of spells, if that. In SWADE, for instance, you're probably not going to start with more than three or four spells (and often less), and it's unlikely you'll learn more than a dozen over the course of the entire game.
If your last mage only knows a couple of spells that have a relatively small number of uses, the game is not going to revolve around them. The other PCs are going to be far more useful in most situations.
Another determinant would be just how well-known this last-mage's existence is or has become. If she just recently came into her talent and only the rest of the party (if even they!) know about it, that's far different - and far easier to deal with in play - than if it's known far and wide that Jocasta is the last known mage in the world.
Because you, the GM, are not required to write adventures about it. Maybe you only play in completely open sandboxes, but you have to realize that's not the case with most games.
My point is that it doesn't matter what I write adventures about. The adventures could ignore the last-mage piece entirely and yet most of the time it'd still be top-of-mind for all involved: "We have to protect Jocasta, she's the last mage." "Does anyone in this town know what Jocasta is, i.e. how much care do we need to take?" , and so forth.
The GM is also going to have to give some thought as to how - or if - Jocasta's abilities can increase or expand in step with the rest of the PCs. The improvements among the other PC can be easily explained in the fiction through training, practice, and so on; but any new abilities or spells for Jocasta are probably going to have to come to her spontaneously...which, depending on system, might force the GM into using or devising mechanics she otherwise might have not.
That player was a jerk then, because your character decisions are not up to other players to force. Especially not with mind-rape magic like charm person.
Yes, it is. This is bad behavior. You shouldn't be tolerating this at your table.
In this case it wasn't just the one player. That, and I'm fully in the "do what the character would do" camp both as GM and player; and when what two (or more) characters would do is diametrically opposed then I've no problem with that being sorted in-character to whatever extent is required.
So forcing a player to have their PC stay at home and forcing a player to continue playing a PC they don't want to are both OK, but actively choosing for your own character to act as a bodyguard is not OK? So, you're fine with things that affect others as long as they don't affect you, personally? Wow.
Er...you might have missed that the PC that was forced to continue play was mine; which means it did affect me.
And the bodyguard piece comes back to doing what the charcacter would do. If I'm playing a warrior-type who's at loose ends and looking for a place in the world and you present me with this task/duty then sure, I'm your bodyguard as long as you need one. But if I'm playing a free-thinking scout type with her own goals and ambitions and who doesn't want to be tied down being someone's protector then you can - how did that old song go? - take that job and shove it.
And that would be dumb move on your part. It's no more sensible for you to go adventuring then it is for a last mage to be adventuring.
Sure it would. In the end (and if it comes down to it; ideally, it doesn't), as a more ordinary warrior or scout (or whatever my function is) I'm far more expendable than is the only one of something that any of us will ever see again. Your bodyguard PC is a perfect example: in that role, if things get desperate, my job as bodyguard is to take that metaphorical bullet and sacrifice myself so that the last mage can survive.
Which would also be a jerk move, because part of the game is to work as a team, not to betray each other like that. And again, maybe your games are filled with people who don't understand or care about concepts like teamwork or consent, but most games are not like that.
Some characters work well in teams, others don't. Most often it's a mix. Betrayal, double-crossing, hidden agendae, or just pure chaos (benign or otherwise) - it's all fair game.
And most gamers actually care about their characters to not just shrug their shoulders like this.
I very much advocate not getting emotionally attached out-of-game to one's character(s). Sure, share the emotions of the character while it's in play, but also realize and recognize that it's just a character; and that when (not if!) bad things happen to it you (general you) have to be detached, and take what humour and-or entertainment from it that you can.
To me, perhaps the worst type of player is the one who gets hissy or pouty at the table when something bad happens to their character(s) in the fiction.
My overarching rule: what happens in character, stays in character.