That is a good point. He should definitely have brought it up earlier - or to put it bluntly, he should have talked to her about the problem rather than waited for it to preciptate. That's a life lesson right there.
(In no way intended to refute or belittle your good point, upon revisiting the whole scene, I think that Donna already knew and had no solution either. The characters don't tell the audience this directly; instead, RTD communicates it in that way he does so well, through the characters' actions. The moment they get back in the TARDIS, Donna begins babbling about where they should go next, in the same way that the Doctor babbles when he's trying to either hide a problem or keep the other person from saying something he doesn't want to hear. (Refer, for example, to the same scene the year before with Martha.) This isn't a Donna character trait - it's absolutely the Doctor's, given to her through the metacrisis.
As the Doctor tries to gently broach the subject, Donna gets increasingly desperate to shut him up with babble, until her mind breaks down. She already knew it would happen and was trying to deny it. The other clue is her line, "If it's in your head, it's in mine." She not only knew, just like he did, what was going to happen, but she also knew what he would do to fix it - her first line after admitting the problem is "I want to stay", indicating that she knew he was going to send her back. Thus, she had as much time as he did to come up with an alternate solution, and we can only conclude that she was unable to.
Of course, he STILL should have talked to her directly about it earlier!)
I'm very hesitant to, as a general philosophy exercise, to go down the "what happens after the Doctor leaves" rabbit hole. it's a great idea for, say, a single story, in which something specific happens as a result of a specific action and the author explores the consequences of that. But the general question is too, well, general. The Doctor's had something like 40 companions and almost all have gone on to life after traveling with him. Whatever life they have must be influenced to a small or large degree by their interactions with the Doctor, and all of those what-if questions apply to them as well. For example, Missy could have easily gone after any of them. Any of them could develop issues from things they encountered in their travels or things the Doctor had to do to solve the situation at the time. (In fact, Tegan did - she developed cancer from something she was exposed to when she was with the Doctor. And exploring that made for a great audio.) I really can't see any meaning in exploring the general question of "should the Doctor be held responsible for everything that might possibly go wrong in the future." (Which, of course, is why Ashildr's storyline fell completely flat.)
no subject
(In no way intended to refute or belittle your good point, upon revisiting the whole scene, I think that Donna already knew and had no solution either. The characters don't tell the audience this directly; instead, RTD communicates it in that way he does so well, through the characters' actions. The moment they get back in the TARDIS, Donna begins babbling about where they should go next, in the same way that the Doctor babbles when he's trying to either hide a problem or keep the other person from saying something he doesn't want to hear. (Refer, for example, to the same scene the year before with Martha.) This isn't a Donna character trait - it's absolutely the Doctor's, given to her through the metacrisis.
As the Doctor tries to gently broach the subject, Donna gets increasingly desperate to shut him up with babble, until her mind breaks down. She already knew it would happen and was trying to deny it. The other clue is her line, "If it's in your head, it's in mine." She not only knew, just like he did, what was going to happen, but she also knew what he would do to fix it - her first line after admitting the problem is "I want to stay", indicating that she knew he was going to send her back. Thus, she had as much time as he did to come up with an alternate solution, and we can only conclude that she was unable to.
Of course, he STILL should have talked to her directly about it earlier!)
I'm very hesitant to, as a general philosophy exercise, to go down the "what happens after the Doctor leaves" rabbit hole. it's a great idea for, say, a single story, in which something specific happens as a result of a specific action and the author explores the consequences of that. But the general question is too, well, general. The Doctor's had something like 40 companions and almost all have gone on to life after traveling with him. Whatever life they have must be influenced to a small or large degree by their interactions with the Doctor, and all of those what-if questions apply to them as well. For example, Missy could have easily gone after any of them. Any of them could develop issues from things they encountered in their travels or things the Doctor had to do to solve the situation at the time. (In fact, Tegan did - she developed cancer from something she was exposed to when she was with the Doctor. And exploring that made for a great audio.) I really can't see any meaning in exploring the general question of "should the Doctor be held responsible for everything that might possibly go wrong in the future." (Which, of course, is why Ashildr's storyline fell completely flat.)