> there is no way to migrate messages from Signal messenger from Android to iOS

Talking only about iOS and Signal here. What you describe is entirely Signal’s decision not to allow backups and restores (on iOS Signal made an explicit decision not to allow any sort of backup and restore). Signal only allows transfer of chats from one iOS device to another when both devices are in close proximity. If Signal wanted to, it could’ve asked the user to enter a strong password, use that to encrypt backups and store them anywhere (iCloud or Dropbox or on a Mac/PC).

Many apps provide backup and restore. Heck, WhatsApp, which uses the Signal protocol, recently implemented cross platform backup and restore for chats. So it’s not technology that’s stopping anyone. Signal is just adamant on not doing it because security.