When I switched from Android to iOS this was the number one technical regression, and for years my go-to example of nice things Apple keeps from us. It’s unbelievable how Signal is sabotaging itself here.
Just use iMessage. It's not as secure as Signal since the server has the keys, but it's the easiest way to ensure that the majority of text messages you send are encrypted.
There was just an article that said that 88% of teens have an iPhone. That means that almost all of their communication is encrypted.
It's various levels of encryption that are acceptable depending on your risk level.
I'm mainly concerned about SMS spoofing and mass surveillance. iMessage protects against that. The only way the government can read your messages is by serving Apple with a warrant to obtain your iCloud backup.
If I had a lower risk tolerance, I would disable iCloud backups to improve my security.
I don't think people are only concerned about the government, but rather the corporations that own your data (via storing the encryption key to use whenever they want, to look through whatever they want)
In the end signal and imessage (with backups off) provide roughly the same claimed guarantee here (on a technical level at least). They both promise not to secretly inject additional client keys into your account, (to at least obtain a forward-operating 'wire tap'), but the software you're running is all opaque and could certainly do so if they wanted (or were compelled) to.
fwiw, iMessage is actually E2E encrypted (without Apple storing the key) if you either don't have iCloud backup enabled OR don't enable "Message in the Cloud".
Historically WhatsApp filled in where SMS was harder to use, and places tended to use either WhatsApp or SMS ubiquitously. So the communities most affected by Signal dropping SMS don’t have an existing social need for WhatsApp.