Governments (and a lot of businesses) like to look at software as a one-time purchase, but it's really better too look at it as a liability and an ongoing cost. It'd be better to have a team make continuous, incremental improvements to the system than have "NextGen" last-gen replacement vaporware.
0) no new logic in the old system (stop the bleeding)
1) incrementally wrap old system interfaces with new system
2) proxy to old system and dark launch new system
3) monitor / confirm correctness
4) cut-over
It wasn't simple, but running old+new in parallel on the same production data stream (then verifying behavior against each other) is a surprisingly powerful approach.
Most critically, because it trades temporary maintenance and infra costs in exchange for decreasing risk and flexibility around cutover date.
Kind of like black box TDD, which helps the new system team be much more aggressive with their delivery timelines.
Even infrequent ~5y update lifecycles tend to be extremely painful unless there is substantial investment in treating it as an essential business process. This leads to a "kick the can" mentality that translates to show-stopping amounts of tech debt.
When Let's Encrypt was created it could have issued 3 year certificates. But it didn't because they knew that's a bad idea, the whole point of Let's Encrypt was automation, and if the certificates expire in 3 years you will say "Eh, we'll automate later" and never do the work.
"Precisely. Months of fruitful work. Leading to a mature and responsible conclusion."
Since it takes 30 years it must be a responsible piece of software, polished to the bone.