[frobozz]

Chat (and communications in general) is a tricky field to evaluate for this sort of thing, due to Metcalfe's law and related effects. I also don't think it would be easy for the gap to be retaken (for the same reason).

I use Slack, Facebook, Twitter etc. because I want to communicate with other people who use those services for communication. If I only use IRC and email, I would miss out on some of that communication.

I don't know the various reasons why each of those people use them, and I agree that it is, in part because of some of the things you mention. However, it is also probably also down to marketing. Many people use XMPP via Whatsapp without knowing it, they are just using Whatsapp.

The user accessibility problem is not universal to all FOSS. In my opinion, LibreOffice (MPL) is a superior office suite in terms of usability to MS Office. Audacity (GPL) has a simpler UX than many proprietary audio editing suites.