I once spent 8 months merging a codebase with almost every single function duplicated three times, and all that was required for each (most of the time) was another parameter, one conditional in function, and all 3 could be merged.

So I did that! And at the same time cleaned up the code, fixed bugs, and adding required new functionality, and all while merging in constant change from the main codebase.

After reducing a 1M line codebase to 280k lines, when almost done, after that 8 months, I was asked "how much longer".

My response of "a month for testing" was apparently too long for the new manager, who turfed all those fixes, including the new functionality.

And I wasn't even behind schedule, I was ahead.

Once abandoned, everyone just copy and pasted my changes... in triplicate, into the old codebase.

Which took 3 months, instead of 1.

Bah. A year of my life wasted.