Yes, the challenge for Peepeth is probably greater than that faced by WhatsApp, but not impossible.
The implicit assumption being made in the parent is that the current market size is dictated by UX, rather than something else like a compelling use case for the majority of the population. I'm suggesting that with some UX work and a compelling use case (which Peepeth may or may not be), there is nothing preventing large scale adoption of crypto.
Peepeth could do something like Whatsapp, say "it costs you 1USD/month to use Peepeth, but for the first year we'll waive the fees" and transparently purchase ETH on your behalf behind the scenes. But they don't do that. They put all the crypto burden squarely on your shoulders, with no hint that they'd consider making it all seamless/ transparent for the regular user.
Keep in mind that Peepeth is a self-funded effort by one person (Bevan). I'm sure he'd be more than glad to solve the usability problem, but purchasing ETH on your behalf can't be done without funding or a source of revenue.
However you can donate see https://peepeth.com/support