"Why is it impossible to send large messages without encryption, despite the developers' recommendations within the application?"

[WofWca]

No problem! See

Large files support

Hi, Playing with Delta Chat I realize that files sent using the app will have the same limitations as attachments for emails, which mean probably around 25 MB or even less if some of the participants has lower quotas. Maybe an integration with services like Firefox Send, which also locally encrypts files could be useful when you try to send a file bigger than 25 MB. See Firefox Send client API implementation for reference.

Short answer, IMO: unlikely, or not soon.