Duraludon is literally the last thing that should have gotten an evolution. It is just blatant power creeping.
Megas were FAR more elegant. However, stuff like Mawile and Sableye should had gotten a regular evolution.
Don't make the 'mon with stat total on par with Rhyperior, Swampert or Lapras Little Cup viable. Scyther was enough for that.
In fact, Megas were a very elegant solution, and should have coexisted with regular evolutions. Then again, regular evolutions had already really questionable crap going on back in Gen IV already. And plenty of Megas were either far too optimized for strong stuff, or not enough for weak stuff.
But have you seen how poorly optimized Duraludon is when you compare it to Pokémon with similar BST? Between frail 50 Special Defense, situational to effectively useless non-hidden Ability, 85 Speed not enough for a sweeper and too fast to be in optimal Trick Room teams, and it’s signature Ability locked as a Hidden Ability until Ability Patch is introduced in Crown Tundra, it do feel like it relied on it’s Gigantamax way too much to become useful on it’s own. And GF did felt this way, hence giving it an evolution when Duraludon become available via Indigo Disk.
If Little Cup were to return in official Pokémon tournament, chances that the likes of Scyther, Duraludon, and maybe even Dunsparce and Misdreavus will be still disallowed from use to preserve the spirit of Gen 2’s Little Cup.
I do have an issue with how Mega Evolution is distributed, not just given to too many popular Pokémon but at the same time, low BST Pokémon are single stage or second stage where they would benefit more of a regular evolution in hindsight. We know GF can break the unspoken rules by fans, and I know many people who wouldn’t mind seeing regular evos to the likes of Sableye in fangames where Mega Evolution isn’t available. As-is, it’s not too far from reality to imagine Mawile and Sableye to finally get an evolution for games where Mega Evolution isn’t possible, with the caveat that such evolution is unavailable for games where Megas are available for use, allowing for rotation.
Gen IV cross-gen evos were an issue because of the quantity, not just because of the quality of some of them. It forces GF to slow it down from not introducing altogether in Gen 5, only introducing one in Gen VI in favor of Mega Evolution, and once more no true cross-gen evos in favor of introducing regional forms in SM, though the Alolan form of Raichu, Exeggutor and Marowak carries this spirit.
SwSh introduced us an unexpected one with GF experimenting with regional forms by way of a few regional form exclusive evolutions to test water to see if fans wanted to see more true cross-gen evos again. With the generally good reception with most of the regional evolutions, they give another try for true cross-gen evolution concepts in Legends: Arceus and SV, this time with lower quantity similar to Gen 2, with much better reception for most of them this time around.
If you want a clearer and true example of blatant power creep, Mega Evolution just did that. While Mega Mawile, Mega Kangaskhan and Mega Lopunny are greatly appreciated, they ended up damaging the metagames in a comparable manners to several non-special Gen 8 and 9 Pokémon thay got banned from OU and that strained VGC to an extent. And let’s not get into how obnoxiously overpowered Mega Lucario and Mega Gengar are. Mega Rayquaza, and Primal Groudon through Primal Reversion, can be argued to be the straw that broke the camel’s back for those who are already iffy with Mega Evolution.
It’s part of the reason why GF is hesitant to bring the mechanic back so far even with the ever high demand, with the last presence in traditional mainline games being Let’s Go Pikachu + Eevee. The other reason is the mechanic’s heavy lore connection to Kalos and Hoenn, thus Gen 6 as a whole, not unlike how Z-Moves, Dynamax + Gigantamax and possibly Terastalization being only in one generation.