I think this will only affect US users, because nobody uses SMS outside the US. And switching between apps is the expected thing to do when trying to push people to another platform, I have telegram, signal, whatsapp and Element on my phone, this is why the new digital markets act is going to be revolutionary, especially with bridge friendly platforms like matrix.org.
This is not true at all, at least for Czechia. The number is going down but it's still in billions (for a country with population of ~10.5M). Quoting from the official annual report:
> In the number of SMS messages sent from mobile networks in 2018, CTU estimates - in the context of the increasing popularity of OTT messengers (e.g., WhatsApp, Facebook Messenger, Viber, etc.) - a slight decrease relative to 2017, approximately by 2% to 8.21 billion SMS messages.
https://www.ctu.eu/sites/default/files/obsah/stranky/284221/...