What supporting infrastructure would have to change? Jetways are compatible across a wide variety of planes. Yes, you can't use the same jetway on an A380 as a 737 (the A380 is just way too big), but you certainly can use the same jetway on a 737 as on an A320, and many other planes besides.
The reason they didn't want to do a clean sheet redesign of the 737 was to avoid pilot recertification, not because of ground infrastructure. Airports are already used to servicing dozens of different types of planes. Adding one more to the mix wouldn't materially change anything.
You sure? The A380 is so much larger (especially in width) that it can't fit into the narrower gate spacing typically seen for smaller planes. Sure, maybe one in isolation could service an A380, but ten in a row in typical airport spacing can't service ten A380s.
Separately, there's the issue that the A380 is so large that you need multiple jetways (at least one for each level) to efficiently load and unload everyone. Maybe you could slowly unload an A380 by debarking everyone off the lower level and forcing everyone on the upper level to walk down the plane's internal staircase, but this is extremely sub-optimal. In practice airlines don't do this; they use specialized gates set up for the A380's special needs.
That doesn't mean it can't use the same _jetway_ (we were discussing jetways, not gate spacing) you'd use on a 737 or that a 737 can't park at a stand wide enough to fit an A380.
The reason they didn't want to do a clean sheet redesign of the 737 was to avoid pilot recertification, not because of ground infrastructure. Airports are already used to servicing dozens of different types of planes. Adding one more to the mix wouldn't materially change anything.