To clarify, side-loaded apps are only signed with a certificate valid for 7 days if you're on a regular, non-developer Apple account. If you pay $99/yr for the developer account, that is increased to 1 year.
There are workarounds being worked on for this - one being AltStore [1] (does not allow arbitrary IPAs yet).
You can also sideload on Windows/Linux with Cydia Impactor[0] - on Mac you can use either impactor or xcode/CLI tools to sign IPAs.
A few minor corrections: AltStore does allow sideloading arbitrary IPAs, even on the non-Patreon version, and IIRC Cydia Impactor is currently broken for free accounts.
There are workarounds being worked on for this - one being AltStore [1] (does not allow arbitrary IPAs yet).
You can also sideload on Windows/Linux with Cydia Impactor[0] - on Mac you can use either impactor or xcode/CLI tools to sign IPAs.
0: http://www.cydiaimpactor.com/
1: https://altstore.io/