Signal is not a company but a non profit, and their service is not proprietary but fully open source including the server side.
That being said, it is centralized and so less resilient, it can be taken down more easily. So you have to pick between more secure (Signal) or more resilient because decentralized (DeltaChat).
Theoretically Matrix has both, but at the moment it is not as secure as Signal, and its UX is clearly worst. And to that you have to add the complexity of decentralization for normal people: which server to pick, how can I know if someone I know has an account... Here the comparison with email should help but still it is not as easy as entering a phone number and immediately you have all your contacts available.
But, and maybe I'm stating the obvious but it is a critical difference, open source is nice but much inferior to open interoperable standards.
Signal-the-company does not allow any clients other than their proprietary compiled client (I believe they sort of tolerate some, but not supported). So while in theory I could use the open source software to run my parallel signal-protocol network, it won't interoperate with the one run by Signal-the-company which is where most people are. So, not actually useful.
Contrast this with email which is an open standard. I can run any SMTP server I like and any MUA I like (or even write my own for one or both), and interoperate with the whole universe of people who use email.
