iMessage between iMessage users is akin to Whatsapp/messenger -- not actually SMS/MMS. Those are only sent when, say, messaging an Android user or if an iMessage failed to deliver.
So it was better to use wifi and WhatsApp to communicate. Once you'd fixed your net access, you could be sure of cheap easy text communications.
The EU roaming pricing law has now fixed the original cause, but it is too late to change people's behaviour.
Here we also have unlimited texting in all but the most limited pay-as-you-go offerings. The telco market is extremely competitive.