Sorry if my post wasn't clear, let me try to clarify.
From a competitive standpoint, I feel that Game Freak already exhausted most options in trying to make Charizard viable. By giving it a Mega it was able to get a huge stat increase, a type change (in the case of Charizard X), and new abilities. There really wasn't any other way for Charizard to get all the buffs that it did if it didn't get its megas.
Audino on the other hand is a single stage pokemon which didn't really have to resort to getting a Mega to get similar buffs. Now that it does have a mega, the prospect of Audino getting a regular evolution isn't likely, which limits any future buffs it can receive. If Audino needs another notable stat buff, or a type change, or a new ability, etc. it won't be able to get those buffs b/c those changes usually occur when a pokemon gets an evolution. Now that it has a mega, a regular evolution is likely not an option.
As for the pokemon you mentioned, I personally wouldn't mind if they had gotten regular evolutions since I feel it leaves it open for Game Freak to get more mileage out of them in later gens. Although I certainly wouldn't have buffed them to the level they did since they would be regular evolutions and not megas.
Hopefully that clarifies it a bit. Perhaps completeness is not the right word, but basically, I see a mega evolution as a sort of final buff or last resort. Ideally, they should only resort to giving a pokemon a mega if it already has had most of its options exhausted. Once a pokemon gets a mega, there's not much to go from there (except perhaps getting an expanded movepool).