I don't think Facebook's apps work when there's no access to DNS. At least it didn't seem like it when I was working to keep that capability for WhatsApp as it moved into FB datacenters.
I don't think very many other applications will work without DNS either, although I never did much competitive testing.
Sure, they could, but at least while I was there, there was no interest in doing it, and amazement than anyone else would want to (and push back on declaring at least a handful of IPs as stably allocated enough to be included in app downloads).
The last 100 times were in the past few hours, roughly. Are you assuming nobody uses FQDNs or URLs anymore? Better yet, are you assuming only humans use those?
I dictated the domain for our home automation system to our early-20s cleaner ("dictionaryword dot dictionaryword"), and after a few minutes she asked me "what do you usually Google to get there?".
The internet is IP. It is very robust. DNS is an application over the top of the Internet.