I think you overestimate people. I told my wife to install Signal because she needed a password for something and it was way to complicated for her to remember. I know what the signal app is and could likely avoid fakes - she would not. I think it is often the case that only one party of the conversation is security minded, while the others just trust that person.
> And the ressouce-abuse. Can this really be a thing?
You know NTP, the protocol for sharing what time it is? That gets abused badly [1]. If you have an open service, there are ways it can be abused. This would without a doubt lead to DDoS-like resource abuse where lazy clients don't cache things properly and just hog server resources. There are ways to limit things like that- but they aren't always simple. Also, like I said before, Signal isn't in the API business.
[1] https://en.wikipedia.org/wiki/NTP_server_misuse_and_abuse
Not limiting freedom of the user because they know what they are doing at the expense of others is maximizing the protection and freedom of a small group, not society as a whole.
Even if that wasn't wrong, it would be a fatal limitation for a social app which relies on network effects. Even if you were actually super-humanly capable of not making mistakes you'd end up using the apps that everyone else you know is actually on.
In this particular case, not likely. People who are into more secure communication do not randomly click on anything. They know what they are doing, or get it installed from people they trust. And if they don't - their fault. Not Signals.
And Signal can continue to work and introduce breaking changes whenever they want. They simply only support the official build of Signal. Any person using anything different, cannot complain, if things stop working. (they will anyway, sure)
And the ressouce-abuse. Can this really be a thing? I don't know in detail how the protocol works, but what can I do with the servers, I can't do with Signal anyway? Sending (encrypted) data from A to B. I can allready abuse that today, if I want.