The problem with most of the airports in the US is the runways, not the terminals.
The number of runways, the configurations, the length, determine how many takeoffs and landings can be handled every hour. You can improve how many people you get through the terminal, but fundamentally, how many people you get in and out of the airport depends on the runways.
You can upsize the aircraft, to some extent, and that’s up to the airline that is servicing the airport. This is why Boeing built the Max, so that companies could get more people on one airplane, and still fit that airplane at the gate in smaller airports, but that upsizing has limits. Many airports cannot fit larger airplanes. You won’t see anything big at LGA.
O’Hare is a notable exception. $15 billion to rip up and rebuild all the runways. The airport now works a lot better. More takeoffs and landings every hour. They can handle the biggest airplanes.
But many airports in the United States are completely landlocked. Take Newark, for example - Jersey turnpike to the east, rail lines to the west, I-78 to the north. You’re not getting any new concrete in Newark, ever, so fundamentally the number of takeoffs and landings at that airport will never get better.
Because of cost, environmental concerns, people suing over the noise, and other reasons, very few new airports have been built in the last 30 years. Denver international is one of the few airports that we’ve actually built in the last few decades. It works really well, including in bad weather, because it was designed and laid out for efficiency, instead of being on the edge of a city, that then encroached and constrained the size.
Six long runways, sufficiently spaced out for simultaneous approaches in bad weather. Ramp space to handle cleaning off snow and ice. Best running airport in the country.
But that success can only be equaled by building airports from scratch. Most airports have been around for a long time, and their size and efficiency is constrained by what happened around them since they were built.
Laguardia made perfect sense in the 1930s, when passenger Aviation was largely flying boats. Pan Am needed water for their Clipper Ships to do takeoffs and landings. So, LaGuardia, Boston, San Francisco, these all made perfect sense. Then the jet age came along, and the runways had to be expanded into the various bays, and Boston was moderately successful. San Francisco was successful, although they cannot build again because of environmental constraints, but LaGuardia just required too much work.
La Guardia is stuck with two short intersecting runways. Takeoffs and landings have to be timed because they’re using intersecting runways, so that limits the number of movements per hour. On windy days, the crosswind limit for small aircraft, like the regional jets, precludes the use of one of the runways, and LaGuardia turns into a traffic nightmare because they can only use one runway.
TL;DR - LGA will never get better operationally because the runways themselves cannot be improved.