How do you debate the "it's more complex in my opinion" statement?

So, some months ago I was looking at some code which has stuff as 300 lines of code function(s) and I could feel the bad smell irl...

I analyze it a bit and there is a lot of stuff which is misplaced, repeated or unsafe.
I first re-arrange it and remove redundancy, then break it down in about five functions (plus a caller), all is now readable and assignIcon k(made-up name) only assigns an icon, it doesn't also send a rocket in space.

But then I put the code in review and the previous author of the code says that it's now unreadable, because s/he has to look as multiple functions. I counter by showing how s/he does not need to read 300 lines of code to find a bug, but approximately 60, and I point at how misleading having an `assignIcon` function which also sends rockets in space is.
The counter? "But it looks confusing to have smaller functions, revert it."

How would you debate that? I am shy and hate myself a lot, so I have issues debating good points, but I am really really sure a lot of bugs I encountered were due to stuff like this so I would like to be able to explain my point in a more efficient way, for future teams.

You guys upvoted a lot my previous rant and this makes me partially sorry, because it means you either witnessed or lived a situation like mine!

So to all the devs out there dealing with an awful work condition, an awful manager, awful teammates or awful wathever: stay strong and remember: you are not married to your company nor you are forced to stay there! Search for a better place, because you deserve it! (And also because happy devs make better code, if you don't believe on yourself and want one more excuse!)

*the Company closes a project and splits us in different teams*

Me: *tells the manager for half a year about feeling extremely bad in the new team which is mobbing me, caling the previous project "shit" (it was not, it simply didn't need to be alive anymore cause we found out cheap alternatives) and not letting me do anything*

Company(half a year later): *sends me into a new project* we don't get why you are underperforming lately.

Me: *full burnout after half a year of being treated as living shit* yeah. Wonder why.