Notepad go-to use was to "clean" the clipboard
I'm sorry what? Why are you opening an app to clean your clipboard? If you need to strip short text just literally paste it into any text field, heck paste into the windows search field does the job. If you're cleaning paragraphs of text then it sounds like you're pasting into something that supports paste-special. Notepad isn't a tool for this, literally any windows textbox works equally for this.
Nothing more, nothing less. For every other usages there was a better tool
Actually it was more. Plenty of people use it to open text files, or make quick edits. You may not, that doesn't mean people weren't using it for its actual namesake. You're right, other tools are better, which makes me wonder precisely why you didn't predict this.
The question isn't why is Microsoft doing this, it's a question of why did it take so long for them to make this incredibly obvious change of developing Notepad further.
When microsoft execs wonder "why are people not happy with out products?"
Precisely why would any of them wonder that? Have you seen their share price and remuneration package? They have literally no reason to question their current product approach, none what so ever.
It is impossible for most of us to understand how far removed people making these decisions are from the real world.
To be fair, this is Slashdot. It's impossible for most people here to understand the real world as well, everyone here is very far removed from how the billion other people out there use their PC. I mean the other day someone complained that Windows always needs messing with Powershell to fix something, that will of course be news to the normal users who don't even know what Powershell is.
Slashdotters are not normal. We're power users, neckbeards, BSD / Linux running nerds. Most people here are detached from the reality outside of this. You as well if you think people only open notepad to "clean a clipboard" (a phrase that already makes most users scratch their heads).