I'm A Banana
Potassium-Rich
'When you're a player, you've lost some of your feyness and become humanoid."
That's just garbage, and a dumb, shortsighted, destuctive approach to game design.
So exactly at what point does a goblin PC 'lose their feyness'? Do they notice? Does anyone else notice? If my goblin PC has an NPC identical twin brother who he grew up with, was apprenticed to the same wizardly master to learn magic, etc etc - what happened to cause the 'loss of feyness'.
This is one of those rules design decisions that is destructive to the integrity of the game world. It introduces all sorts of immersion-breaking questions and inconsistencies. If the rules descrive the world poorly, you should be changing the rules, not the world.
I think there might be a bit of nuance lost here.
When I read the first sentence, I read "players can't be fey," which feels deeply limiting. But the idea of there being forthcoming species that aren't humanoid implies that there's nothing inherently taboo about non-humanoid PC's. That's good. Non-humanoid PC's are an important bit of player kit in many settings. I'm inclined to believe that while a humanoid species might have lost some feyness (ie, the existing goblin PC option), you don't HAVE to lose some feyness to be a PC goblin. You can also have a goblin species for PC use that is fey, and that'd be fine, too.
What I'm hearing overall here is more like.... that "goblin" and "fey" are two circles in a venn diagram with some overlap. But "goblin" can overlap with other types, too. Fey, fiend, humanoid, and aberration are the ones that stand out as likely, but the idea of a goblin with, say, the Beast type...well, it certainly sparks the imagination (wereworgs!). And when expanded beyond goblin, you could imagine, say, githyanki with the Dragon type (infused with DRAGON BLOOD), or orcs or goliaths with the Giant type, etc. And these may or may not be PC options depending on what kind of product we're referencing and the creature's role in the setting, etc.
So your typical old school humanoid goblins still exist in the world. But also, the goblins in the MM are Fey that emphasize that Feyness. They can exist alongside humanoid goblins, without changing those humanoid goblins. Both kinds are there.
To me, that's pretty smart design, especially for an edition change that's backwards compatible. Allowing a diversity of definitions, while emphasizing the one that works best for the stories you have in store, is a pretty clever way of validating those who are keeping things unchanged, but also charting a new trajectory that you're excited about.
Not that I'm not still salty about other GLARING OMISSIONS with 5.24. But this particular element seems better thought out than it might seem on the surface.