Have zero issues with recasting anyone as LONG as the character is needed. The Waltons didn't seem to thrive with fake John Boy, but I dont watch the series enough to gage it.
DALLAS had done the recast many times when a guest spot turns into a permenent and the actors says no or is busy- Like with Morgan Fairchild who stupidly said no. And 1st Gary... Jock would have been no different, just find an actor with the same build. Tucker would have been no good either as he died in 1986- same year as Donna Reed

Its better to lose the actor than the entire storyline. Jocks' demise didn't hurt the series like Bobby (only because of the idiocy return).
Yes, sometimes a certain character is need to maintain the show's narrative consistency, and a recast is necessary.
The choice that a show has to not recast, and the entire dynamics of the show change it into something else that is not as good and leads to a quick cancellation,
or recast and at least the show has a chance to continue.
We are all familiar with Darren. When the recast occurred, the show was already on the air for several years, and sitcoms do tend to have a
somewhat limited time. I think that most people would have preferred Dick York. I thought that DY's Darren seemed more like a bully, not letting Samantha be who she is. I always sided with Endora on that point. Dick Sargant's Darren may have also wanted Samantha to not use her powers, but he seemed less intense,
less dictatorial about it.
It may have been considered that Samantha would meet another mortal, with Darren meeting a tragic end (hopefully not of Endora's doing)
It could have been interesting if the next mortal was much different from Darren, but the tendency would have been to rewrite Darren with another Darren-type anyway, so they might as well recast.