Whether it's $9 or $150 I can assure you the project would go over budget and cost three times the estimate. That's not just the government, that's almost every major IT project regardless of sector.
I worked in telecom as recently as mid 2010's - a HUGE company. We dealt with a myriad of different billing and automation systems...something like 70 different systems. COBOL and CICS on the automation side of things (which would feed various billing systems) were still VERY much in use.
I once asked why and they said the cost to migrate away from it - and actually being able to build something "better" - simply wasn't possible at that time from a cost standpoint.
It worked, so why change it?
They may have finally switched away from them by now, but my guess would be to actually do so probably caused a lot of headaches (and cost) to do so.