As much as the fan in me would enjoy seeing old Doctors get new episodes, I think it would be a terrible idea for the show.
Mind you, as a child I was somehow convinced that Peter Davison had come back for the Matrix scenes in Terror of the Vervoids (I hadn't really seriously started watching it at that point, and Saturday afternoons were usually spent being dragged around town by the parents, so all I saw of Doctor Who, or the Tripods, was what I could glean from dragging my heels near the telly shops. Still, I was hooked).
and thanks to Time Heist
And I think it only got away with it because the whole thing was more of a skit.
"But fair play to you, not a lot of men can carry off a decorative vegetable."
Ah yes haha, curse you series 8!
Yes, given it was just a for CiN, it is taken as canon for how old Doctor's can appear I suppose.