Email is decentralized. Most people (including me) decided to sell their data for a better service which is why we ended up with Gmail and a few other services that are very popular. If Google and the other services are forced by law to not be allowed to do that we could easily just create an account in the thousands of not millions of email providers that offer alternatives, if Signal goes down, we all have to go back to WhatsApp.
Even in the hypothetical case of this law leading to google not providing email (neither of which I think would happen) you'd just end up on some other server run by some other corporation with all of the risks that entails.
You could argue that's better than trusting one service (debatable imo), but you're still a thin client vulnerable to the whims and data protection capabilities of that provider. Even worse, if everyone is using just 3 or so main providers then since you're communicating with people centralized primarily on these other services you're also vulnerable to their security practices too.
All of this is because of a few core problems with the current stack. Linux servers are too hard to run. Modern dependency requirements are a mess, operating systems were designed without an eye toward the web or identity so spam is a problem. You can't fix this without thinking more deeply about these issues, I think Urbit might be a way out. Without fixing these issues the options available in the current system are niche communities or centralized services. The idea of people running their own decentralized nodes at scale given these constraints is dead on arrival. PGP is a failure - even when people's lives literally depend on it they cannot or will not do it.
Trust me, I wish this wasn't the case - but I care about winning more than I care about idealistic purity. You have to think pragmatically about what the best option is today and what crazy changes are required for this to succeed in the future. Today with the existing tech stack I think Signal is the best we can do at scale in competition with whatsapp, fb messenger, and iMessage. Tomorrow I hope urbit gives us a way out.
it does not, see Email