I don't think them falling behind is necessarily bad. Modern systems, languages and programming practices are much nicer, but they also have a lot more effort behind processors, compilers, frameworks, operating systems, etc., so overall the system is much more complex.
If anything, I think having a separate, archaic, mostly frozen system base for critical infrastructure is a good thing. The current ancient COBOL abandonware situation is probably not the simple and mostly standardized solution one may wish for, but it's a lot closer.
I'd agree, if nothing ever changed and it was feasible to run them forever. But it's not. At some point (probably 2038) the whole thing will fall over and a new system will have to replace it.
Fortunately we still have 17 years till then so surely all these companies have started preparing for this inevitability already, right? (i know the answer)
If anything, I think having a separate, archaic, mostly frozen system base for critical infrastructure is a good thing. The current ancient COBOL abandonware situation is probably not the simple and mostly standardized solution one may wish for, but it's a lot closer.