NL is rather small sample to, say, larger countries like Poland that use SMS quite frequently. And depending on carrier, SMS texting most likely will be completely free with most of the current plans.
Signal's rationale is just Signal's own reluctance to build an umbrella messenger. And given they do drop SMS, still won't introduce usernames it's very hard to actually sell it as a WhatsApp replacement.
And now, with WhatsApp supporting password protected cloud backups and up to 2Gb attachments, I'd say Signal will loose the userbase it acquired during the hype and Musk tweet.
In fact, during 2020 Belarus protests, Signal did nothing to support it's own operations during internet semi-blackout in the country, while Telegram tweaked their server side to provide at least some possibility to know what was happening in big cities.
So what are the values of Signal — I don't even know. But they sure did support pillagers and rioters in the USA.
To be even more brazen, Signal is not Apple. They stopped innovating. And they don't have enough political power to convince people do things the new way.
Even their zero knowledge server is worthless. Check out the story on FBI cracking down on the leader of some right wing proud boys type of armed group. They tracked him and then compelled to give up access to Signal.
Their innovation stopped at providing solid cryptography that was adopted by most decent messengers already. And they aren't visionaries with cancelling SMS.
UPD: the funniest part is that the service that drops the SMS support still relies on SMS to provide account registration.
This is just an unprecedented level of sarcasm.