Have to agree, that's why Uk carriers offer unlimited SMS on there tariffs.
It's because most people wouldn't use them, but they cap/limit data as it's far more popular with iMessage, whatsapp, etc
I think out of my contacts only one person uses regular SMS as there reluctant to upgrade there ageing nokia.
If SMS are 'free' (inclusive) and data is not, then only a fool would send texts via Whatsapp or iMessage.