Maybe eventually, it has to do with market share and if the service is a “core platform”. Signal doesn’t have enough market share to warrant it yet, even iMessage wasn’t forced to since it’s not that popular in EU. The law was mainly targeted at WhatsApp as that’s THE messenger in the EU.
You can if the company is a non-profit like open AI. Basically when you take on investments for a company you declare what the goal/purpose of a company is, either to make money (for profit) or for some other nebulous cause (non-profit) eg. Ending hunger or saving humanity from AI. If an investor thinks you aren’t following that goal and are pursuing some other goal then they can sue the company.
Sadly most companies are for profit so they can only be sued if they’re trying to do something that doesn’t optimally make money. So a fossil fuel company can’t be sued for legally dumping poison into the air if it’s the most cost efficient method, but they can be sued if they do a less cost efficient solution that would make air quality better because improving people’s health isnt there goal, making money is.