Why the limitations?

- only 45 days of media

- link lost if inactive for 30 / 45 days

- max 5 devices

Is it because of reliance on the network to provide these features?

I used ICQ decades ago and I could back up everything and move it to a different computer, I can still search the message archives to this day, and there weren't these silly artificial limits. Why has chat regressed?

This isn't completely true, while linked devices probably can't do everything, they do act as independent message endpoints. Messages fan out sender-side, are encrypted and sent separately to each device of the recipient [1].

[1] https://github.com/signalapp/Signal-Android/blob/9b2c2ed66d8...