Boeing still builds 737s and the older ones are upgradeable - Southwest has both bought new ones and rebuilt old ones with many upgrades in recent years.

F40s were a stopgap at the time and would be arguably illegal to use today (from both air quality and crew accommodation viewpoints, not to mention fuel use/greenhouse gases to be politically correct). Not to mention being just plain ugly (yes, I know, the GEs are even worse!) and too small/short range to be of use in most operations. And of course practically everybody is out of the passenger loco business (Boise notwithstanding) so there's no practical rebuildability/upgradeability for F40s short of custom jobs. So Amtrak made the only decision they could: buy GEs when they were available. And the states and commuters made the only decisions they could: buy F59s (F40 on steroids, or with a funny nose) while they were around, or whatever custom job they can find since EMD gave up on passengers completely. I don't know who would or could respond if Amtrak were to try to replace all the GEs now.