The true crime here is MoP at 5. That expansion was easily an 8.Let's look at Metacritic scores:
TBC - 8.0 user score - generally favorable reviews - subs rise
WotLK - 7.7 user score - generally favorable reviews - subs rise
Cataclysm - 5.6 user score - mixed or average reviews - subs decline
MoP - 5.0 user score - mixed or average reviews - subs stagnate
WoD - 5.9 user score - mixed or average reviews - subs decline
Legion - 7.3 user score - mixed or average reviews - subs rise
BfA - 3.1 user score - generally unfavorable reviews - subs decline
You could make an argument that Legion does not fit the pattern since it had mixed or average reviews, but 7.3 is a lot closer to TBC's 8.0 and WotLK's 7.7 than any other expansion.
The fact is that these reviews are broadly representative of the state of the game. Thousands of people can't all be wrong about something. MoP had good gameplay but unappealing story / design direction so subs stagnated. Cataclysm, WoD and BfA were all badly received and coincided with a rapid drop in sub numbers. Legion was generally seen as a return to form for the game and although we didn't get exact sub numbers, it was pretty evident that they were on the rise just judging by increased activity across all endgame content.
All of these people can't be wrong.