The reality is that for certain actors who imbued their Doctors with distinct personalities, there are things like the Tom Baker effect - no one can play him quite like Tom Baker. Still, David Bradley portrayed the First Doctor, a role originally held by Hartnell, in Peter Capaldi's final two episodes as the Twelfth Doctor, as well as a cameo in Jodie Whittaker's run as the Thirteenth Doctor, and was quite highly-regarded for his work. (It's worth noting that Bradley also portrayed the late Hartnell himself in the BBC drama/docies An Adventure in Space and Time about the show's origins.) Bradley stepping into Hartnell's role years later opened the TARDIS to a maelstrom of possibilities, with multiple Doctors returning in various ways. Multi-Doctor stories will always be popular, and if done well and with consideration for Doctor Who actors who have ed on, they can be both entertaining and respectful to their memories.

Doctor Who Recasting The Second & Third Doctors Would Set A Bad Precedent

Recasting the First Doctor should remain a one-off event that shouldn't be a regular thing whenever a Doctor Who actor dies. Doing so sets a bad precedent by potentially detracting from the legacy of the previous actor who has become associated with the role, and possibly ruining their character with writing that changes their personality. Recasting in this format would also confuse new fans who haven't been watching the series for long, not to mention irritating longtime fans who had been viewing since the beginning and become accustomed to associating a particular actor in the role they helped make so distinct and memorable.

This can become a problem with any series that has the track record of Doctor Who. Not only that, but there's so much inconsistency across the seasons due to changing writers, budgets, and showrunners that recasting the actors would cause even more inherent turmoil. That a series can survive with its main character being played by different people every season is testament enough to its staying power, but if it becomes a common practice to recast characters fans have already seen, it could dismantle the infrastructure that's made the series beloved for so many decades.

The Only Way Doctor Who Could Recast Patrick Troughton & Jon Pertwee

If Patrick Troughton and Jon Pertwee were to be recast, then there's only one logical way to do so effectively and respectfully; the second and third doctors should be played by their sons, who look enough like them to make their appearances work. Michael Troughton already played the second doctor for "Big Finish" and Sean Pertwee, an established actor in his own right, is a dead ringer for his father. Not only will it appeal to die-hard Doctor Who fans, but it will also prevent confusion for new fans who might choose to go back and revisit the beginning of the series.

Most fans have their favorite Doctors and, barring the nature of the time lord's character, are devoted to their memory. Many fans have grown up with a Doctor associated with their formative years or a time in their lives when the series was particularly meaningful. Honoring the memories of the Second and Third Doctors isn't as simple as recasting them with any actors available to take the part, except in rare cases where they would have very little screen time or old footage could be used.

Doctor Who Already Has A Better Way To Include Old Doctors

During Steven Moffat's era, First Doctor footage was reused to appear new (such as getting in the TARDIS and meeting Clara). While this wasn't a perfect solution, it avoided any questions of being disrespectful and looked fairly convincing for what it was trying to achieve. With modern technology, there are definitely ways to do this with the Second and Third Doctors if recasting proves to be too difficult. It would also save unfavorable comparisons between dDoctors, particularly if the writing on the page didn't represent them accurately.

There are also wonderful ways to merge look-alike actors with the likeness of other actors, though that gets into matters of rights and their estate. While it's been done well within the Star Wars Universe, it might be more difficult for Doctor Who to pull off. That, and the series doesn't have the same budget as Disney and Lucasfilm, which makes the conundrum of whom to recast all the more salient, particularly since fans have been critical of the last few seasons.