Who are those experts?
Further, there is the implication in your comment that Android and iOS are less secure than some other format (assumedly a linux or bsd variant desktop OS) for secure messaging. I'd be shocked if you could find a group of security engineers that even agreed with that. You certainly will find lots of them that disagree, so its in question at least.
I'd go further and suggest that if I made the proposition that from a security perspective you were best served by using e2e encrypted WhatsApp purely from an iOS or Chromebook that I'd gain more agreement from the security community than any other proposition about messaging formats other than "don't use electronic messaging for secret things".
Yes, being able to update their software at will enables them to read messages that have been already sent, even with e2e encryption.
> Further, there is the implication in your comment that Android and iOS are less secure than some other format (assumedly a linux or bsd variant desktop OS) for secure messaging.
Endpoint security is a can of worms. There is no simple conclusion to be taken from my comment, except for the literal meaning.
They very likely don't, but that doesn't mean they can't.