I've said this a million times before but it bears repeating.

If you've got a complex but working highly available distributed system and you're trying to replace it with something new that's "simpler", you'll spend 6 weeks on designing the replacement, 6 months on implementing a working prototype, and 6 years on fixing all your edge and corner cases that the old system had already addressed that you were too dumb or arrogant to even consider.

And sometimes the number isn't 6, but 10.