I'm sure others have pointed this out. I'm just a bit slow sometimes...
One of the things we saw TWICE in S4 is that the Doctor can be cloned, once as Jenny (apparently a fully-fledged Gallifreyan with two hearts etc.), and once as a semi-human clone of himself, lacking the ability to regenerate but in other respects the same man.
So the next time they want to regenerate him (which takes us into "Contradicting earlier canon" territory, admittedly something they've never worried about too much) they can instead "discover" that there's traces of the Doctor's genetic material in that clone-o-mat, or open up the rift between the parallel worlds and yank back the semi-human version and give him an upgrade to full Time Lord status. Both of which reset the clock on his regenerations and give the BBC a lot more choice for future Doctors; for example, they could do the female Doctor thing for real if they wanted to.
Wonder what Joanna Lumley's up to these days...?