How much would it cost Microsoft to buy Notepad++ and Classic Shell, and remove two of the biggest annoyances in modern Windows?

Meaning, of course, that they would use them in Windows, so there would be a text editor and a start menu that work. Not buy and kill.

They did it for sysinternals

for notepad++, they already have vscode

Plus, I think Windows strives to not be "batteries-included" in some ways, perhaps in order to encourage a healthy third-party software ecosystem (and perhaps to follow the spirit of the anti-trust ruling.) Windows 10 has fewer bundled apps than Windows 98 did. Nothing is an "optional feature" any more; it's all App Store packages [that happen to be made by Microsoft] now.

Windows 10 still has plenty of "optional features". Two ways of viewing these - Settings/Apps/Manage Optional Features (For instance, if you want to install openSSH, this is where you'd do it). Also, Control Panel/Programs and Features/Turn Windows Features On or Off. This is where you'd install the telnet client, container support, Hyper-V, tftp, etc.

VS Code seems like less of a general-purpose text editor, though (and is also much "heavier")

heavier sure, general purpose I'm not convinced

Not really comparable in term of resource utilization