I can’t tell you why the only engine options on the table seem to be ‘new JT8s or retire the plane.’ But I am sure the engineering studies that would go into other engine alternatives at this late stage would be far too late to meet the need.
And with a a 40 year old airframe, new wings and fusalage/structure repaires will be needed in the not too distant future.
JSTARS is a real antique. As long as we plan to continue to put a manned system up for enemies to shoot down, the most logical choice, IMHO, would be to move the electronics suite into a larger, newer civilian airframe. It’s not like there aren’t plenty of newer airliners around.