Honestly the fact that Mega Evolutions were reduced to an afterthought in Gen 7 and straight up removed in Gen 8 really suggests to me that Game Freak has their regrets about how Megas were ultimately executed. The concept wasn't bad in theory but the execution was very bad both competitively and in-game (in-game they just completely trivialize the game) and I'm sure they probably realize that at this point.
Regional forms and normal evolutions are the far more survivable "changes" to older Pokemon that are executed better. Regional forms granted are more of a YMMV, and come at the expense of potentially rendering the original form more obsolete, but they can in many cases breathe new life into an older Pokemon without completely making it overpowered, while on the contrary in many ways they still make older mons more interesting with the regional forms. New typings, potentially new abilities and movepools, and perhaps even slight stat changes that don't overpower the mon too much make many older mons have a new way to be fun. Alolan Sandslash and Alolan Ninetales are two of my favorites in that regard in that they take totally different directions from their original variants, but the OGs still have their place. Seeing some of them get new evos with Galar was a nice spin on the cross-gen evo tradition too. Not to mention regional forms are much more interesting in terms of flavor. Megas are just an artificial transformation that is temporary. Regional forms have nice flavor in that they come off as adaptations of a Pokemon that is still a Pokemon, but became different from its originally discovered form to adapt to a region it was in. Like how Ninetales, Sandslash, and Darmanitan all adapted to a colder region. Or even a case like Galarian Zigazoon, which is speculated to be the original Zigzagoon, but as they came into Hoenn they became friendlier and lost the ability to evolve into Obstagoon. Or even differences in personality, like how Galarian Darmanitan is much friendlier and calmer than its Unovan form, while the reverse is true for their Zen Mode forms. Or how Perrserker is more violent than Persian who is snobby, and the two mutually dislike each other. There's a lot of room for more interesting flavor there with regional forms.
Of course, normal evolutions themselves already stand the test of time much better anyway. They were a good way to bring new life into older Pokemon, with the likes of the Johto and Sinnoh ones still standing the test of time today as they buffed old Pokemon well enough, and the ones from the regional forms in Galar are also pretty well liked and will likely continue to survive. Look at the likes of Kingdra, Electivire, Magmortar, Yanmega, Steelix, and Togekiss for instance. Or the likes of Sirfetch'd and Obstagoon. The fact that we're still getting regional forms even after their debut in SM, with both Galar and Hisui bringing forth new ones and even new evolutions to older mons, shows that these are here to stay and we will continue to get them in the future. And honestly, that's for the best.
Normal cross-gen evolutions and regional forms will inevitably in the long term stand the test of time better than Megas ever will.
