You certainly should worry about the possibility of zero day attacks either when using FOSS or a proprietary solution.
If the provider of a proprietary solution stops offering it you might have no easy path of migration where as hypothetically FOSS solutions are easier to self host or find a new hoster for (and thus reducing the cost of a potential migration)
If a FOSS solution is abandoned, it's doubtful that most companies are going to take on the maintenance themselves. They are going to start an initiative to migrate.
But that goes back to the old saying "no one ever got fired for buying IBM." The same can be applied to Microsoft or AWS. Yes MS has put plenty of technologies in maintenance mode but you didn't have to rush to migrate. IBM still sells system that are compatible with what they sold in the 80s.
On the other hand, if a FOSS solution is abandoned you typically have quite a long time to migrate since you can self-host. If AWS shuts down tomorrow and you rely on it heavily, you're basically fucked.
If AWS shuts down tomorrow. There are a lot of companies that are going to be in the same boat - including Amazon and Netflix.
I would be more worried about my one colocated data center getting shut down or getting destroyed than the entire global AWS infrastructure that has data centers in 50+ availability zones.
As far as software, most AWS services are just managed versions of available software. Even if you're using lambdas for API endpoints, if you treat your lambdas like Controller actions, it shouldn't be that much of a heavy lift to concert them into standard MVC APIs equivalent.
For another perspective, listen to the podcast interview of Adrian Cockcroft, the guy who led the migration of Netflix's infrastructure from self hosted to AWS hosted.
If the provider of a proprietary solution stops offering it you might have no easy path of migration where as hypothetically FOSS solutions are easier to self host or find a new hoster for (and thus reducing the cost of a potential migration)