In short, the wave is very much that two sentient beings can fall in love despite biological differences and procreate, because with magic/science/the Force/whatever, all things are possible.
Which is the problem with presenting half elf and half orc as separate from other species mixing is to tacitly imply they are "acceptable" pairings in the games rules, and somewhat to imply that other options are frowned upon. It's what the game does now; you are at the mercy of your DM to allow a given combo to exist and what kind of stats it will produce, unless your choice is a human/elf or human/orc, then the game has blessed those unions with unique species traits and deemed them explicitly allowed.
This is an interesting take.
Part of the reason I don't really like the Fantasy genre anymore, is that "anything goes". It makes it hard for me to suspend my disbelief and feel some kind of relatability to the world. Perhaps it is a generational thing, as I think today's generation doesn't like being fixed by boundaries or constraints, and that's why D&D has become more and more appealing (in the 80s and early 90s, D&D's dominance wasn't like it is today; my best friend's father had a hobby store and I was privy to sales numbers). In fact, I sometimes wonder if the younger generations are so disgruntled with anything resembling the real world, that they actively want their settings to be as unfettered as possible?
Perhaps part of the problem is that D&D is setting agnostic, while also trying to maintain some kind of lore. But how can you really have lore when world X can have something contradictory to world Y in the multiverse of D&D? The problem is when you change the lore of setting X over time. Instead of appropriating setting X and changing it, why not create a new setting Y with the new background and rationale?
One of the few fantasy RPG's that is still kind of interesting to me is Runequest. Its almost 50 years worth of lore for the same setting is unparalleled, and the Bronze Age feel is more interesting to me than the fantasy-clockwork-steampunk that Pathfinder has gone, or the superhero-fantasy D&D has seem to become. Runequest's world setting hasn't needed any real retcons (that I know of) for 45+ years.
That's the kind of language WotC explicitly wants to avoid, and unfortunately that means half-elves and half-orcs lose their privileged status as the acceptable hybrids. It's a generational thing that WotC is going to want to get ahead of.
If any ancestry is biologically compatible with any other ancestry, then sure...go ahead, mix them as you please. But at least also somewhat mix up the physiological differences. A liger is not 100% a tiger or lion and a liger is really "just aesthetics". Same with a mule. For that matter, even mixes within the same genus (think dogs of different breeds) have different physiological characteristics. This is why mixed ethnicity people like me find it somewhat racist to be boiled down to just one ethnicity or the other. Acknowledge the differences (whether physiological from a game perspective, or cultural, from a game or real world one).
I also understand that accounting for mixed game mechanic advantages is more challenging from a game design perspective. Instead of 1 or 2 compatible mixes, you now have N x (N - 1) possible mixes (where N is the number of ancestries). But in the real world, only species (and below) can interbreed. At least for me, there needs to be some kind of rationale for how any ancestry can intermix with every other ancestry.
Even if WoTC wants to hand wave it away with magic and say all ancestries are biologically compatible with each other, it still becomes somewhat offensive to say "well, yeah, but you're really just 100% of one ancestry or the other...it's just a game after all". Then if the logic is "it's just a game", why change the term races to ancestries? Why remove that certain ancestries are inherently evil? They are tacitly admitting that the game
does touch on real world matters and can influence how gamers see things
in the real world.