SimpleX

I assume ease of use and spam prevention.

I think Signal tries to be at least somewhat attractive to the average person who wants more privacy than just using WhatsApp or whatever. Making it easy to message existing contacts helps a lot with adoption.

One of the design goals is that they don't have a user database, so governments etc can't knock down their door demanding anything. By using phone numbers your "contacts" are not on their servers but local on your phone.

To prevent spam and to allow people who already know each other's number to easily contact over signal. If you want an anonymous account use an online sms activation service paid with monero, personally I recommend smspool.net .

as I see it, Signal tried to fit that privacy gap for a standard centralised messenger, if you think about it, that might have made it easier to non-tech-savvy people to adopt it (even if it was as a request from a contact), decentralisation is not remotely appealing to them

Signal is not perfect but we control its app, libre software. See SimpleX Chat.

Escaping WhatsApp and Discord, anti-libre software, is more important.

It's focused on ensuring there is no middleman between you and the other party, but it does not have a goal to provide anonymous messaging. Sadly.

If you want to be mainstream a) you can't have spammers, scammers, and all the other scum of the earth and b) finding your contacts in the app HAVE TO be plug and play. Literally no normie will bother adding with usernames or whatever.

Because it's centralized, I prefer SimpleX.

Maybe I am being too simplistic here. But I have never received a spam message to my XMPP account and I don't know how a spammer would find it.

In a phone-based system a spammer can spam a list of numbers, or use contact lists that are easily shared via phone permissions. There are several low-effort discovery processes.

For e-mail, you get spam when you you input your personal e-mail into forms, websites, or post it publicly.

But for something like XMPP... It seems rather difficult to discover accounts effectively to spam them. And, if it is an actual problem, why not implement some kind of 'identity swap' that automatically transmits a new identity to approved contacts? A chat username does not need to be as static as an e-mail or a phone number for most people.

I just don't see 'spam' as such a difficult challenge in this context, and not enough in my view to balance out requesting a phone number. Perhaps a spammer can chip-in?