So I feel like I'm missing something.

If you already bought the product, you can keep it, and get to keep using it. No one is going to stop you, and it'll still be on D&D beyond.

If you haven't bought the product, you can buy the newer version that represents the current best practices of monster design.

This seems like a good decision to me. This way newer players aren't buying all 3 books and realizing that the older 2 are just the inferior version of this new book (Imagine the outrage at paying for 3 book and realizing that 2 of them are just redundant). I'm literally not seeing any downside. If you like the old product you get to play with it. If you have a DM that suddenly bans it, that just applies to that DM (and they could've done that anyway). The only potential people affected are new players who didn't buy the old book and now can't. And yet, those aren't the ones posting here about it.