I imagine that anyone who hits their quota and wants to add more would start making room by deleting their trash. I can't imagine there are many instances of people increasing their quota just so they can keep stuff in their trash around, and if there are, these people will probably just move the stuff out of the trash so they can keep it after this change.

More likely, Google will be deleting vast quantities of data and saving money due to the reduced amount of storage they're using.

Even though trash counts towards a user's quota... users are, by definition, using less than their quota. And the percentage of users paying for increased Drive storage is likely a small percentage of overall users.

So deleting trashed items will mean Google is storing a lot less data, and probably not impacting people's storage purchase decisions much/at all. I expect it'd be a net benefit for their bottom line.

Trash items do count against your quota.

Yes, as of today, trash was counted against the storage limit.