The trouble is that doing the same thing over and over can get boring; both for the fans and for the band.
Doing the same thing over and over has worked for AC/DC, but I think they are the exception rather than the rule. The bands I have followed for decades have all progressed in their music. Had they not, I probably would have gotten bored and they may have to.
Consider Rush. They have had distinct periods, each punctuated by a live album (first album being the exception - an era of 1 album). The first period was the side long epics. Great stuff, but I wouldn't still be listening if they hadn't gone beyond that. Then there was the new wave era. Very interesting at the time, but again I would have gotten bored. Certainly throughout all the eras, you could hear what they were listening to in their music, but their music always came through. Progress while being true to themselves.
Even Metallica, after doing 4 albums essentially the same, they found a need to move on. They lost some fans because they "sold out" (friends don't let friends cut their hair) but they probably gained more than they lost.