With a few exceptions the AoS playtesters are all solid players and decent humans. They aren't miracle workers though and ultimately work with what GW give them.
Personal perspective, one issue is that some factions have historically been strong on the competitive scene and some playtesters may be more invested in those. For instance, DoK and Seraphon have been top end for most of the lifespan of competitive aos. Most top end tournament players own one or both of these armies, even if they don't play them all the time.
Anyway, I'm not convinced an internal playtesting team would do a better job for AoS. If it was up to me (and money no issue) I'd have an internal and external team, like how videogames have external testers.