If you think 4-7 factions will be "up there" - then the game isn't obviously "solved", and there is a relatively healthy meta, even if a number of factions don't have a build.
I mean to my mind at various points of 8th, you had a solved meta. You were imperial soup (Castellan+Guard and then probably, but not always, some BA or Custodes).or Eldar Soup (often flyers and some leveraged Ynnari).
Then from middle of last year, you basically had Iron Hands, Ravenguard Successors, or an SM Soup taking advantage of IF LOS ignoring shooting.
Which isn't to say other lists couldn't win - you had for instance the possibly exaggerated Tau panic when Siegler won Nova - but the numbers were so skewed in favour of the above, not taking them was probably foolish (see... Sieger again at the LVO.)
Through the edition you had many lists which would win one major tournament, and then disappear - often because these were toughness lists, which were always vulnerable to an early unlucky streak costing you a game, and statistically that's just likely to happen at some point (and does, hence they don't repeat their success despite people emulating the list.)