Opening up Rat would help refill the resorts (at the expense of locals still holding onto APs).
That’s my question: would Rat opening draw more people to travel to WDW and stay in the resorts? I don’t really know (it would certainly do it somewhat but the question is to what extent). If it would increase resort bookings and help Epcot hit capacity caps regularly (without negatively affecting the other parks’ attendance) then it would seem foolish to not open as soon as possible. The cost of running the extra rides/area seems marginal in the grand scheme of things.
And when things go back to “normal” hopefully sometime next year, they can still use Rat (and Mickey) as a “new” attraction for advertising given how few people will have passed through the gates to ride them. Plus they’d still have two new big rides opening in 2021, maybe a new night show and maybe even a (borrowed) light parade to advertise. I think opening Rat now to spread that out more makes sense.