I certainly do not agree with that!
For example, Notepad has been one of the buggiest and weirdest pieces of software I used on stock Windows. It does weird things when saving files, like moving your cursor to another location, so if you are typing and saving and typing and saving you will end up with text misplaced all over and a bizarre file that has a weird back/undo history. With its large host of weird bugs I would not expect the current codebase to be simple to update, and surely you'd agree a rewrite of Notepad is more than a few days.
I in no way think that improving Notepad would be one of the easiest changes to make to Windows. Sure, there would be harder things to improve I'm sure, but I don't see why it's a contest at all. Updating Notepad sounds like the opposite of easy to me.
Improving Notepad for everyone who uses it, while not receiving millions of death threats for errors or bugs or unexpected, unwelcome changes in behaviour sounds nearly impossible to me, personally. Perhaps this is part of the reason it is so stagnant - MS sees little upside to making large improvements while seeing enormous risk. Just a guess.