Alright, so from my personal experience (a bit like you, not an expert on Delta Chat or Simplex)

I tried Simplex recently, and I found the user experience a bit confusing. The chatrooms I was part of were quite verbose, and in one of those rooms people were mentioning the battery drain that simplex can create. I also wasn't able to connect to the SimpleX client on Linux for some reasons.

DeltaChat on the other hand is really newbie friendly. You just set your username, and they take care of the rest for you. You can still connect other devices, and even change servers without encryption getting in the way (imagine the opposite of the way Matrix does things)

I see your point about "why use email if you have to reengineer it anyway?", but if we look at other messenging alternatives:

• XMPP also created it's own protocol, and now seems to be in a constant limbo of which clients and servers implements which extension
• Matrix who created a very confusing experience for new joiners who don't want to deal with encryption keys on a regular basis

None of them was really convincing either even starting from scratch.

That being said, if you like SimpleX, then it's probably the best for you. I like Deltachat because it was easy to convert my family to it, something I never really managed to do with any of the other alternatives.