Yeah, but GF is more keen on just forgetting pokemon most of the time than buffing them. If anything that was part of why the decided to do the cut. Can't make all of them useful at once, so ditch half of them (but then they still kept releatively useless ones in like Basculin...)
As for regional varients, they are a very mixed bag, because too many of them chamge the pokemon so much that they are basically another pokemon with the same name. So what happens is that technically an old pokemon might get a new chance in the spotlight, but only by giving up everything they were. Like Ninetales didn't get better in gen 7, it just had it's name used by a drastically different pokemon that had a few similarities. PLus stat totals stay the same, so the best that ever happens is a variant might be a bit more min'maxed. Regional varients are for the most part a lateral move rather than an improvement (with a few exceptions like Muk)
Now not all mega were great. SO barely changed how the pokemon worked (same typing same ability) and just made it more powerful. But the really good ones gave new life into certain pokemon that were never going to see much use ever again. PLus they allowed for something that never existed before, ability combos. Since many megas changed abilities, some pokemon got some really cool bonues from it. Mawile and Gayrados could now use intimidate to get a free debuff and them mega evolve to make use of new abilities. To go back to Lopunny, you could now use a limber Lopunny and switch it in when you were expecting a thunderwave. Now yuo got a free switch and could them mega evolve to a semi sweeper with perfect type coverage.
And when it comes to Dyna/Giganta not being as strong. it might make it better for balance, but it kind looks silly to have the enormous building sized monsters then getting one shot like they were their regular size. Like from a lore aspect they should be more powerful than megas just because they are supposed to feel like these overblown Kaiju battles. While we are talking about lore/design, megas were also more personal to the pokemon that got them. They looked at them and said "what is this pokemon about and how can we accentuate that to it's next obvious degree?" and because of that Mega designs were far more varied and unique. Alakazam is all about it's mind, so it's mega has a withered body. Kangaskahn is all about a mother taking care of it's baby, so it doesn't change at all while the baby grows up a little.
Every gigantamax form has the same design goal of "emphasize something that's remarkably big about this pokemon" and yes, they found different ways to do that, but they all lead to the same end goal. The only Gigantamax thar actually feels personal to the pokemon is PIkachu because it is a reference to Pikachu's origins.
Dunno, it's just they seem to be trying to make such a big deal (pun really not intended) about Gigantamax when it just feels rather underwhelming in actual practice. instead of a replacement to Megas it more feels like a buff to z-moves where you can use three of them (if you don;t get ko'd before then)