Didn't RTD rule out the idea of ending Sarah Jane's story in the
My Sarah Jane tribute that was aired on CBBC? Did I just imagine it when he said that she would always be out their adventuring? Continuing the series without Lis Sladen would have to deal with her absence at some point.
One way for RTD to not end Sarah Jane's story, like it or not, is to simply continue the series with a new actor in the role of Sarah Jane. Face it, while Lis was a big part of what made the show popular, she wasn't the
only reason for its success. It's a sure bet that CBBC doesn't want to see this popular offering come to an end, if there's even the slightest chance that it might prove to remain popular, with a different actress playing Sarah Jane, or (preferably) replacing Sarah Jane herself with a somewhat analogous character.
I bet CBBC is really putting the pressure on RTD and Company to find some way to keep this popular show on the air in one form or another.
So like it or not, the show might well go on with a different actress playing the same role, or, in a wholly new but comparable character.
And while no one can "replace" Lis, there are plenty of talented women who could nevertheless do justice to the character Lis played, or a comparable replacement character. And in a genre, as well as a "verse", where even death can and has been fairly regularly retconned as is needed, the means of doing this would be child's play.
Within and without the Whoverse, replacing actors, or adapting a new character to take the place of another, is fairly commonplace.
Wayne Rodger's opts to leave "M*A*S*H", so "Trapper John" is written out, and Mike Farrell's "B.J.Hunnicutt" is written in. Likewise when Harry Morgan's "Col Potter" replaces McLean Stevenson's "Col Henry Blake", and when David Ogden Stiers' erudite "Chas. E Winchester III" takes the place of Larry Lindville's bumbling "Maj Frank Burns". All of these characters were beloved (including Frank) and yet all were not only replaced, but successfully replaced, to the point where many believe that the replacements were superior to the characters they replaced.
Mandy Patinkin wants out of "Criminal Minds" so "Jason Gideon" is written out, and Joe Mantegna's "David Rossi" is written in. And again, while Patinkin's character was much beloved and incredibly popular, so much so that most felt the show wouldn't survive his departure, the show actually (and unexpectedly)
increased in popularity after a very brief period of adjustment.
"Bewitched" never even acknowledged the fact that they recast
several roles, both major and minor, (Husband Darrin, Nosy Neighbor Gladys Kravitz, & Larry Tate's wife) for a variety of reasons (including death) during the series' 8-year run.
"Roseanne" not only replaced a couple of actors and characters in it's 9-year run, but always did it in a way that was both funny, and totally in character with the series.
Nearly a dozen actors have played The Doctor. Likewise, at least half a dozen have portrayed James Bond, most within the same continuity, and without any the benefit of a plot device like "Regeneration" to "explain" the character's sudden change in appearance and manner. (As far as I'm concerned, if Desmond Llewelyn is "Q", it's the same damn continuity.)
Replacing an actor/character can be done, and done respectfully.
Heck, when Heath Ledger unexpectedly died right in the middle of filming "The Imaginarium of Doctor Parnassus" director Terry Gilliam found a way to rewrite the story so that other actors could take over Ledger's character,
mid-film, and still allowed Ledger's performance to be used/seen, and did this in a way that not only worked within the confines of the story's subject matter, but which (arguably) enhanced and improved the story.
John Thaw passed on before all of Colin Dexter's "Inspector Morse" books had been brought to the screen, and rather than let the remaining stories go un-produced, or recasting the role that had become so associated with Thaw, they instead rewrote the stories, adapting them to allow Morse's faithful sidekick, "Lewis" (Kevin Whately) to respectfully continue in his mentor's footsteps.
Replace Lis Sladen as Sarah Jane? I hope not, though I have no doubt that it could be done. Replace the character of Sarah Jane? That's a different story altogether. As to whether or not it should be done... that's kind of up to the CBBC. And apparently they don't want to lose a popular show if there's a way it can be saved. And, for the sake of those who earn their living making the show, I hope it continues in some form or another.