[1] >>38117385
Signal also intentionally doesn't store too much data, long term data costs will slowly grow over the years. I imagine for a bigger platform, costs can grow to multiples of the rates for Signal and smaller Mastodon servers.
€10 per year should be more than enough for most users, though, and it should be quite affordable for most countries.
Then again, instant C2C and C2B digital payments using mobile phones is growing extremely fast in most of the global south.
[1] https://www.app.com.pk/national/pta-introduces-9999-sms-code...