It's harder for people to accept when the band has done 20+ years without a lineup change, like STP did.
FNM made the change very early on, Layne was pretty much done within 5 years of hitting the big time and Cantrell was at least equal to him in terms of the band's sound and creativity, Van Halen only did 9 years with DLR, and besides the band was called Van Halen, Not Lee Roth.
Bands like Journey, Priest, Sabbath, they all had revolving casts of musicians right from the outset, drummers, guitarists, keyboardists and later vocalists were all subject to regular change and change back. It was considered more normal to their fanbases.
STP are a different animal, they were always sold to the fans as a unit and made a lot out of the brotherhood angle of being in a band together, and that is reflected in how rabid a lot of their fans are about it. However, I don't feel that way personally, I am quite happy for the band to continue however they see fit without throwing a tantrum about it, although that doesn't mean I'll necessarily like or buy their music.