Evolutions are better than Mega Evolutions because you can balance it and use it on more than one team. If GameFreak makes a crapton of megas there will be more Mega Steelix's where it becomes outclassed by an own mega evolution like Mega Aggron. I'm honestly sick of Mega Evolutions they only should've been for specific final evolutions anyways Mega Absol would've been so much better as a normal evolution.
I'd tend to agree with you, in part. While some megas, such as Mawile and Audino, in addition to the ones you mentioned, might be better off as proper evolutions, there is an inherent issue in cross-gen evolutions that was quite prominent in the Gen IV evolutions, and that Game Freak probably isn't quite looking to revisit, involved in the actual methods that such evolutions would require. Game Freak goes out of its way to avoid implementing evolution via methods that already existed when the pokémon was introduced.
Take Eevee for example. Umbreon, Espeon, and Leafeon were all prime candidates for evolution via the Moon, Sun, and Leaf Stones respectively, but because Eevee couldn't evolve into Umbreon or Leafeon in Gen I, it wouldn't make sense if it could later. This is why so many new methods and specie specific items were introduced in Gen IV.
Now, because of this issue, pokémon who would later be given evolutions could not attain those new forms by normal means. Level and item based evolutions are the most common forms, but neither could be used without introducting a completely new condition first. This means such conditions have to be created as well, be they odd movepool expansions, oneoff items, or specie based interactions - like remoraid and mantyke or karablast and shelmet; say yungoos gets a final form that is based on interactions with the rat that it's tied to, but that rat isn't introduced until Gen VIII, for example. In the end, this just creates a hodgepodge of complicated mechanics and a cache of mostly useless items. This was the aftermath of Gen IV, and since then, and perhaps because of this, the only new evolutionary mechanics that have been introduced for older pokémon belong to Feebas and Eevee alone.
Onto Gen VI, and the introduction of megas. Of the megas introduced, only a select handful of would have worked as proper evolutions. Some, such as Kangaskhan and Aerodactyl would be thematically inappropriate evolutions, while others edged on being too overdramatically powerful or different, even, to properly fit either. Slowbro and Camerupt seem like examples of the awkwardness this would bring. The majority of the megas that have been introduced were for pokémon who literally could or should not have evolved normally in the first place. So in the end, we're left with a small percentage of megas that while, yes, probably would have worked as proper evolutions on paper, only stood to add further complications to the mechanic. It would have been one thing if this was the second or third generation after megas had been introduced, because Mega Evolutions wouldn't be so new and shiny then, but under the circumstances, Game Freak probably felt like they were making the best decision.
