For the 60th anniversary, we may not have Tom Baker anymore, and who knows who else will be gone, especially companion-wise (William Russell is quite the older fellow).
I'm not so sure about recasting the role. But, agree with your point about actors passing on. It's inevitable. And, it was that short term thinking that prevented Nicholas Courtney from appearing in the new series. That's terrible! Yeah, I know that he was on the SJA, but it's not the same.
We should bring back the surviving actors for the TV series while they're still with us. Once they're gone, they're gone. That's assuming, of course, that they want to come back. If not, entirely different matter.