You are correct about the need to be able to get to GCT under exceptional circumstances. That would definitely require keeping a small fleet of 3rd rail dual modes around. Fortunately, with the advance of solid state power electronics it is now almost practical to build trimode engines, and I understand some have just been built elsewhere. But as you said, no matter what, they are technically complex beasts.
As for standardizing voltage and frequency, Amtrak at one time had such a plan, which eventually got shelved given the cost of changing the ~12kV 25Hz system to 25kV 60Hz system for the entire NEC South and SEPTA. The existence of the original plan that prompted NJT to go with 25kV 60Hz when they refurbished the Hoboken Division. OTOH, MNRR just changed the frequency to 60Hz and got rid of all the specialized 25Hz gear but did not change the voltage.