I think that sometimes a character naturally goes a certain way or it's a combination of performance and how you use a character in a certain way. Also it makes sense that canon becomes more nuanced over time as it allows you to tell better stories and ultimately that's Joss's first priority. The vampires of the Buffyverse ARE still different to those of the Riceverse - the Rice vampires are degrees of humanity depending on how they want to be and there are grappling with ideas of how ethical they want to be in their consumption and existentialism as you'd expect with beings with human minds and souls who will live forever. The Buffyverse vampires are corrupted humans - they are the worst versions of the humans they once were with no universal compassion, some have compassion for individuals. Buffyverse vampires remain monsters, with one and eventually two exceptions, but they're more a comment on monstrous humanity than outright monsters. I think in a sense Spike was ensouled because it was the only way for Joss to preserve his worldview but not have to kill this fasinating character.I don't know Joss seems to go back and forth on a lot of what he says.
For instance re Vampires. Joss back in the 90s used to go on about loving Vampires, ran through all the Vampire films and comics that he loved, but then in the 2010s went on about how much he has always hated Vampires, and they were just generic monsters to him.
According to James Marsters meanwhile he hated Anne Rice and was adamant about Vampires being ugly, bloodthirsty monsters and this is backed up by everything about his original script and the first season which portrays them that way. (Notice season 6 which is when he has the least to do with the show is when they are at their most romantic. Season 7 however when he comes back, we get the Uber Vampires, the best ugly, horrible, monstrous Vamps possibly of all time.) Now however Joss goes on about his love for Anne Rice.
Similarly Joss used to go on about how he liked Doctor Who, about how the Doctor is dope, how he respects RTD for bringing it back, but now he says he always hated it.
A part of me thinks he says what he wants people to hear, but others goes against the popular trend. For instance he said he loved Vampires when they were firmly creatures of horror in popular culture, but then in the 2010s when they became sappy, emo figures he said he hated them. With DW it was the same he said he liked it when it was more niche and respected by sci fi fans, but as soon as it became a hipster thing in the early 2010s he distances himself.
I think he likes to be seen as a proper, serious genre fan, but at the same time he will pander to what his most vocal fans like, re Spike. I'm not saying "Oh this makes Joss a coward" or anything. I understand when you have such a devoted fanbase it's difficult to know what to say.