One thing about the vinyl or vinyl equivalent bags like the Ortliebs is the ease of patching holes with clear repair tape. I've got holes in the bottom of my front ones from banging into curbs, especially in Japan where curbs can be high for snowploughs. A bit of repair tape on each side and they're fixed.
I have lightened my ortiliebs a bit by replacing the backing stiffener with thin coreflute. I didn't do it for that reason, I needed to move the hooks to suit the Tubus Duo racks I have. I patched the old holes with vinyl repair tape.