I disagree--The genre was evolving, but the fan base was stuck on the 1990 VGA point and click model and didn't really want to accept anything different, despite several different kinds of offerings
I'm not sure there ever was a 'monolithic' fan base (or any 'consensus')... Almost every 'new' game was drawing in new fans, and splitting others from the 'flock'.
I had several friends who were big KQ fans back when they were first released, that don't care about KQ now, because they moved onto newer games and IPs as technology advanced... Those included some that just 'grew out of' gaming in general, developed different interests. Still those are anecdotal and not necessarily the 'rule'....
The last two official games weren't near as popular as the first six.
Actually, the last two sold more than any previous game in the series except maybe KQ5, KQ5 was said to have seriously outsold KQ6, KQ5 initially sold 500,000 copies and was the bestselling computer game for the next five years after its release (1995?) (so apparently even beat initial sells of KQ6 and KQ7). Roberta once said in an interview that KQ8 sold double the previous two games, and apparently KQ7 sold double of KQ6... A couple of years later Sierra, under Vivendi was even going to make KQ9... What killed KQ really was that Sierra died itself... It lingering death killed Space Quest 7 and several other classic adventure games at the time (and over the following years)...
So keep in mind, if sells were any indication, even when KQ6 was released, the fandom may have already been halved or more, less than when compared to when KQ5 was released, and its initial sells... In anycase we have data from Sierra itself (though not specific numbers for each game), in which they state KQ6 was less successful than KQ5, and possibly even the later games (although arguebly it received better general reviews than KQ5, and KQ7!).
KQ7 had possibly the worst reviews, but it was more successful than KQ6 in sales... KQ8 was more successful than KQ7 in sales apparently, and even Grim Fandango (if Roberta is to be believed), and most professional reviews considered it average to excellent...
KQ1 remake was largely hated by the fans on its release, and didn't sell many copies at all. Most people received copies through later released KQ collections. Though now there are many fans (new?) that like it...
As for the fans? And popularity? Again that's hard to tell, because many of the oldest KQ fans were already moving away from KQ and aventure games in general into new genres... What was left was a 'niche' crowd, who wanted classic adventures to continue on. When the rest of the industry knew they just couldn't compete in the new market.
But seriously, I doubt anyone is going to find honest statistical data out there that would break down history of the 'fans' who they were, where they came from, demographics etc...
Infact I wouldn't be surprised if we are actually the minority, the elite, the most devoted of the fandom! A special breed!