> One can hope the EU will eventually mandate unlimited SMS on cellphone plans, but I don't see WhatsApp being dethroned another way

I doubt it.

Here in France, cell plans have had unlimited SMS for a very long time now. Yet, WhatsApp still is extremely widespread. Now, I'm not the most socially connected guy around, so I may not be attuned to any new developing trends in the matter, but IME it doesn't seem to lose any popularity, and something like 95% of the people I interact with on WhatsApp are locals.