I assume this works like traditional cellular roaming, no? So this would mean you have to be a customer of one of the service providers to get reciprocated roaming capabilities abroad.
I spent ages working out why the “older” part of the house had no cell coverage…
Texting 911 requires you to be attached to a network, which in turn requires a phone plan and potentially a roaming agreement. Changing this would require some deep architectural changes to the 3GPP specs, or just allowing anyone on but filtering out all non-911 texts.
Especially when they are making money continually the whole time by selling everyone's data, even if you could keep the same phone for 20 years.
I know Google is pretty good about fixing security vulnerabilities in the Pixel's baseband, as well as providing Android updates for quite a while, but baseband feature updates seem like a different category still.
With satellite based SOS, there's no guarantee that the local 911 can support texts, or even that there's an equivalent to 911 (think calling from Antarctica or the middle of the ocean).
It's complicated and relatively expensive to coordinate - which means it's unlikely they can open this up for phones without a plan. (Compare with traditional cellular 911, which is all government funded.) The US Air Force RCC does this as a taxpayer-funded service for PLBs, but they don't support any private satellite services.
Long-winded way of saying there's a lot that would need to change on the backend and funding model to make that work without being tied to a paid cellular plan.
That means the real answer to your question doesn’t have a single answer since it’s in the legal and contractual parts of the equation.