First, I'm not seeing crippling levels of supply chain constraints in my industry (steel, concrete, electronics, labor etc.) but I can't speak for what would be needed in a Skyliner expansion.
Second, the existing Skyliner route is 2.15mi from Pop to Epcot + .5 mi from CBR to DHS.
Pop to Blizzard Beach is 1.72 mi if you just go west, and then another mile (2.69 total) to go to AK. So just by linear feet it's roughly the same. Though you wouldn't just go straight over AoA like that. Coronado is another .5 mi if there is a hub at BB (though I doubt they would have a hub here).
I don't see any issue adding on legs to one of the existing lines. I'm sure theres plans for it.
If they started now, 8-12 months of planning/permitting/site planning, 1-2 year lead time on materials, 2 years construction? These times overlap, but I think you can easily do it under 4 years if you wanted to.