Actually at the time KQ8, was actually ahead of most things out at the time. It was praised because it pushed the limits of then technology (every KQ game was known for pushing the technology). It was way ahead of most things on the market at the time.
Technology has obviously bypassed it, since then.
Let's face it I think its a better game than Doom... and people still praise doom... Both were innovative for their time, and both have become dated by modern technology.
If I was going by modern standards, most older games would be mediocre by today's standards. That would mean the entire KQ series... People still enjoy old atari games, and nintendo games, but imo many of them simply haven't aged gracefully, and I don't find much fun in many of them anymore...
Actually, the name on the cover, had nothing to do with KQ8's success, it was said by Roberta, most of the sales were from new players, that had never played King's Quest before. It certainly maintained some of the core fan base, but the core fan base wasn't really all that big to begin with. It was the new players that latched onto it the most, and gave it best ratings. While some of the older players, gave it lower ratings. But in general most critics rated it as 70% or so. Not the best, but certainly not "poor". Let's face it poor games don't get better ratings, for simply having a popular "name" on them... There are plenty of modern series, with entries given poor ratings... Why because the reviewers don't see them as being any good... Having a popular 'name' doesn't guerentee a good rating.
From what I've been discovering KQ7, actually had pretty middling/poor ratings as well, might even be as low as overall 60%, but it didn't draw in as many new fans as previous games. As she pointed out, she had received alot of hate mail for KQ7 from older fans, but she never let it bother her. It might actually be the lowest rated KQ game.
Its interesting to note that two popular adventure gaming sites, gave KQ7, between 40% and 50% (D) rating back when the gave their ratings for the game...
Strangely enough they gave Mask of Eternity, around an 80% (B) and 100%.
Like I said you can never account for taste, and each mileage will vary. But it seems if KQ8 had remained close to KQ7 in style, it probably wouldn't have gotten very high marks by the industry, and probably still would have failed as a 'proper ending to the series'... Either way KQ8 probably still would have been a 'cult classic' of a sort.
Like I said, I think KQ6 is the only game of the series, that maintained consistent high marks across the board. NOthing less than an 80.
Most of the other games, have mixed reviews. People either love them or hate them. You have fans that came in during point and click era, that absolutely hate the parser era. there were fans that played from the beginning but were turned off by the icon-driven systems. they couldn't please everyone.