We've had much more troops in Japan, Korea, Germany for many more decades. The Taliban wasn't able to push into any major populated area with just 2,500 US troops.
Those troops are under essentially no threat with the exception of Korea and that is tiny. Having troops do more than occupy space is massively expensive and dangerous for them.
Keeping troops in developed, desirable countries is probably far cheaper than Afghanistan. Regardless, the point is what is the long term goal of keeping 2,500 troops, or even 1 troop in Afghanistan? There are lots of places in the world mired in conflict and poverty.