Is it possible that Signal don't want to go down this path as it potentially puts them at the whim of the demands or requirements of paying businesses? Which demands/requirements may not be aligned with the degree of privacy which set them apart from other messenging apps.

If not this, could another reason might be that they're still a small team and this, along with the other changes that would presumably need to come along with it (accounts that aren't tied to a phone number), are what are holding them back.