It's a business decision as
@AliciaStella said. If the main SNW ends up on a 10-12 acre spread at the 3rd park, then it's because they needed that as the anchor there. The attractions will likely be the same, but on a larger piece of land with better crowdflow.
Look, if Vivendi had never sold the 1800 acres and instead built 2 theme parks on it, then you'd probably have 3 Nintendo areas being built in 3 of the 4 parks with completion dates around 2020-2021 for all 3 lands. There's no doubt in my mind about that. Universal would not be sitting on the Nintendo IP if they already had a fully built out 2nd resort.
That's the difference between where WDW is and where Universal is. Universal has spent the past 3 years acquiring land (finally appears to be done), and now they're going to finally get the 2nd resort built. WDW can do what they're doing because they already have the fully built out resort, it just needs all the IP now of TSL/SWL/GOTG/etc.
It makes too much sense to move Mario to the 3rd dry park. The last thing you want to end up with is a situation where in 2030, your attendance numbers for the parks looks like this (Mario in USF):
USF: 15 million
IoA: 12 million
3rd: 8 million
I get that fans don't care about business matters like that, but those are the issues that should matter to Universal execs. If Mario is being moved to the 3rd dry park, it's a business decision being made to avoid an attendance imbalance from being created, nothing else. If they don't think the above scenario would happen, then they would build Mario in USF.