I forgot about Toxel! I suppose what I meant was new pre-evolutions of existing Pokemon. But Toxel is cool because it was introduced in the same generation as its evolved form (like Togepi and Riolu before it), which makes it kind of special in my eyes.
I miss cross-gen evolutions in general, actually, and hope that we see more of them one day. The new "regional evolutions" are a cool idea, and I love the Mega Evolutions, but it would be nice to see some regular evolutions too. It could be that Game Freak just stopped doing regular cross-gen evolutions because they wanted to keep things fresh and surprising. I think we've now reached the point where regular cross-gen evolutions would be surprising, because it's been a while!
I think it's because there's no practical purpose for Baby Pokémon to exist unless they were introduced alongside their non-baby forms. They're good for marketing, from a business standpoint, as they're meant to be cute versions of pre-existing Pokémon, and initially, they ensured that breeding was required for anyone who wanted to complete a Pokédex, but the former could be done with regular base forms of new Pokémon, as well as regional forms (notice, for instance, that Galarian Ponyta is physically smaller, has a larger head, much larger eyes, and stubbier legs than Kantonian Ponyta), and the latter has been rendered pointless ever since they dropped the rule of "Baby Pokémon cannot be found in the wild."
That, and it might have initially been to try to rekindle hype for Pikachu, Jigglypuff, and a bunch of other Generation I Pokémon, but as a concept, it couldn't really last.
Don't get me wrong; there are some Baby Pokémon I quite like, such as Munchlax and Toxel (the latter of which may well be the very first traditionally "cute" Poison-type that isn't also Grass or an Ultra Beast--though one can argue that Poipole might be a Baby Pokémon too), but for practical purposes, Baby Pokémon of previous-generation Pokémon like Azurill and Budew are essentially new versions of existing Pokémon with worse stats, and for the majority of them, require extra steps to breed. (Wynaut was the one valuable one for a while for being Wobbuffet's only means of having Encore, but recently, Encore was added to Wobbuffet's level-up moves, rendering Wynaut redundant.) Would've been nice if all Baby Pokémon found in the wild had three perfect IVs like they did in Generation VI; I don't know why they scrapped that.
I think Baby Pokémon from previous-generation Pokémon can work if they're 1) pre-evolutions of very strong, imposing Pokémon with high stats that didn't evolve from something, and 2) can be found in the wild in the early- to mid-game. They've already done so with Munchlax and arguably Happiny. Lapras would indeed by an example, as it's too strong for the first half of a Pokémon game, but introducing a Baby Pokémon version of Lapras can be a means of giving you access to the evolution line earlier into the game without making it overpowered. Duraludon is another example, and for these reasons, if they make more such Baby Pokémon, one ofr Duraludon would be at the top of my list. (Most of the ones introduced in Generation II, however, were definitely made for marketing and to show off the then-new breeding mechanics.)