Out of all the classic Doctors he's the biggest and thus would be the easiest to sell to people outside the hardcore fanbase and thus make more money. On another note, BF has the license till 2025. I know this is pretty pie in the sky stuff but seen as how we're already talking about recasting people who aten't dead yet. Do we take it as read that it will be extended or could that be the end of BF at least as we know it. They could be struggling for new stories by that point. Colin has just got his own "final" story and McGann is now working toward his. As we've said the old guard may be getting thin on the ground by then so could they either loose the license or give it up. Maybe keep making "non-Doctor Who" audio for the likes of Charlie Pollard or Bernice Summerfield along with actual non-Doctor Who stuff like Blake's 7 or original stuff.
As far as I'm concerned, McCoy's the only Doctor that could feasibly have stories told for an extended period of time, because of the loose ends presented wherein. Second would be Colin, because on audio, he still hasn't met Mel yet, chronologically speaking. But the rest... I dunno. Sure, Tom's fun, but are there stories for Four to say that haven't been told already? Five's run with Adric doesn't interest me in the least, and Eight is basically a couple of box sets shy from ending his era for good. I'm actually glad we're getting more Lucie stories, instead.
As much as I enjoy all of them, I wouldn't mind them sunsetting Davison and Colin. Syl has a lot of potential (and one-off companions they keep peppering him with) still to mine at the end of his timeline. (Something they actually were good about doing early on, but of late it seems they're sliding back to using the companions available which is limited the far end. I want a good, solid end to him and Klein, for example.) McGann seems ever closer to a "final" story which would seal up his time, maybe give him some "time off." Meanwhile, every two years they can bring him in and add new Charley, Lucie or Liv stories in between what is there. I can definitely see a transition away from the Monthly range soon to boxsets and more emphasis on the extraneous titles. As we noted, Moffat more or less gift wrapped Clara/Me for Big Finish and you could easily do Bill/Pilot adventures. Plus, 11 and 12 are untapped markets for the most part, and if they can get them in that would be the cream on top. The only boat that seems like it won't come into port for them is Eccleston. They were able to get John Hurt!
There's new B7 audios coming out next year (full cast) but even there the cast it getting on which could limit how much further they can run. Gareth Thomas is gone, Jacequiline Pearce is gone, Paul Darrow's 78, Jan Chappell's mid 70s and Michael Keating early 70s. Steven Pacey's the baby of original run - he's only in his 60s.