It's not that David Bailey should be considered irreplaceable, I was just thinking about where Sam Groom would have the most resonance taking into consideration his past with both the audience and the cast. If Sam Groom had previously worked with Victoria Wyndham or any other actor still in the cast I think it would have been easier to have him cameo as Russ in the finale. Or if they had had him for more time and it had been a meaningful storyline for Josie that could also have worked. Of course if it had been a real storyline they could have recast Russ altogether.
Thinking about very brief replacements, I have to admit that I found the brief run of Stephen Bogardus as Sandy Cory a bit jarring. I know he was there because it seemed important for Maggie's father to be present, but as I recall he was just a conventional corporate and stern father without any hint of the louche Sandy.