You are correct re SBAS models, and I did mention older models. Diminishing yes but that could take many years.
The idea is to streamline flight planning, bring it together into one place - much like NOTAMS and Weather is already. Yes you can do it on the device, but that lengthens the time during pre-flight/startup, time better spent elsewhere. And you would have to do it for each location an approach is required including up to 2 alternates (worst case).
Also, not everyone can afford to upgrade their GPS units, either immediately or at all. For example, updrading to a SBAS enabled GTN650 (x2) will cost most owners ~£30k. If you already have an older non-SBAS unit, theres little reason to upgrade.
So, this is about time and financial savings for pilots/owners.