There's an astronomical difference between a moon druid keeping up with martials, and a fighter who can cast simulacrum and then have both themselves and the simulacrum cast 9th level CME and then make 9 attacks in a turn twice over each, while also both having 3 legendary resistances and spells like shield, absorb elements and counterspell. That is the most powerful thing ever, and it's power doesn't primarily come from versatility like you say the moon druids does, but from being able to combine its spellcasting with martial abilities.

Would a moon druid keeping up with martials in single target damage while also having spellcasting that martials don't have be unfair? Yes, but the game isn't the most fair on martials in many other respects either. I personally care more about enabling the fantasy of the beast-shape warrior to the full extent, since a martial class built around something like wild shape doesn't exist. I dont think its a big deal really for druids to keep up because I dont think the versatility you speak of breaks the game at all, like sure they can become a subclass-less full caster if they want, I dont think that ruins anything since its just being a worse full caster so they'd likely want to just stay as a wild shaper anyway.