I work in a giant place, 100k+ employees, 150+ years of documents, layers upon layers of processes, presence in all countries, heavy regulation, successive mergers.
There is simply no way to make it work perfectly. We can do migration upon migration but that has never seemed to simplify anything (the joke in my team is as soon as something starts working, we migrate half of it away to a non working solution and end up with two half systems - this has proven true so many times we dont even take it as a joke anymore).
We embrace the chaos, and focus on concrete client problems, and collect the fees for service well provided. No point in trying to make it make sense anymore or to micro optimize productivity. We have a rough estimate that it takes a year for an experienced new joiner to be truly productive, and we simply pay well so they don't leave before doing their first big contribution and basta.
I work concurrently on two identical systems doing the same things on paper but with important difference in details, because the current political war made the american system win over the asian system, while knowing the asian system simply handles better our 13 market quirks than the american one that was made for a simple single market. We slowly complexify it to reach feature parity, deploy it in markets with low stakes while we improve the asian system continuously for the hundreds of high-paying clients. One day either we'll reach a point where a market that matters can finally be migrated, or the fact we make much profit in Asia will catch up with the global strategists and the asian system will start being proposed to solve (badly) the american problems.
It has never been thought by our strategists to keep two systems working well in their respective core markets at the global scale. So we migrate half of Asia and keep two systems working badly at the local scale.
I pity the intern joining in 1000 years though. We have no idea how the kraken will evolve after centuries...