The problem with RTD's whole big tragedy idea is that for a tragedy to actually feel tragic instead of frustrating or predictable, you have to first give some tangible hope to then get crushed by means that follow logically.

So if they'd had another season (or hell, even compressed more character development into the one season they had), and Donna hadn't gotten screwed out of everything meaningful to her like that, and the Doctor had successfully gotten over some of his demons, and then they went for the big tragedy and IDK, had Donna die epically or something and had all the Doctor's character development unravel due to that trauma and set him up to crash and burn on Mars, that would have been tragic.

But yeah, how it all actually ended was like "what, are you kidding me??" and made me feel like RTD cared more about his agenda and ideas than his characters. Which was doubly disappointing because one of the things I always loved about RTD was that even when his writing was tone-deaf or shitty, he always, always seemed to love his characters.