They were also significantly limiting how many guests would be allowed in the Parks, which also means lines were short already, so its not quite the same.
The closest I've seen it to having a full park and no FP is when the app/FP had an outage for almost a day, this was prior to 2019 if I recall correctly so it was when Max Pass/regular FP was still a thing. And it had negligible positive affect on Standby that day.
So we'll see if whatever this new service, if it gets released, will actually do to the lines. I just don't know if this service if it replaces LL will have the desired affect on Standby that some thinks it will.