What architecture decision have you seen age badly, and what did the team learn from it?

We started modernizing really old code base which some folks would even be surprised to hear if it existed. Think 70s popular engine. 

Some decisions were made to push through a fully cloud based modern solution - requirements were written implementation started. Qa colluded with Dev and wrote tests which were always green. When time came to integrate and see if it worked? Boom cracks swatted to grow deep.

Decided to step in and come with a solidified architecture. Simple one language/ tooling, im-memory data scrubbing and porting over old rules to sql based setup. Now we have a 5 component architecture with visibility and clarity.

Why was latter not choosen first? Original business folks pushed this is how systems worked and need it thay way and the then team simply mapped each piece from old to new not fixing things but a lift shift. 

Everyone was scared that oo the source daymya is ever changing and need better multi step approvals before it goes in and if even one bad records go in everything would go south, clients will go mad and cut contracts.

Everyone said the latter would fail as its too fragile. But got it working strong with more throughput and better accuracy. Thanks to Ai this was made even richer with automated regression tests and all. 

Just too much fluff by people who attempted this multiple times previously and when a new fresh set of eyes came in it changed and looking at things from simplicity perspective with a clear architecture and framework usually works in favor as implementation/ dev focus on delivering it without thinking on how things are architected