I'm not a Windows 10 user yet, so forgive me if the functions I mention are not in Windows 10. I'm assuming that you did an in place upgrade to Windows 10.
Have you tried turning off all Windows security functions (within reason, defender plus any others that can be turned off), running MC17 first, then running the uninstaller in Windows 7 compatibility mode? Actually, set the shortcut to run in compatibility mode, and then run it as Administrator as well, using the right click method. Sometimes changing the shortcut to run as Administrator isn't enough. Nor is being logged on with an administrator account.
If MC17 won't even run on Window 10 for you, try to run it as Administrator as well, right click method. For a MC version to uninstall properly, it really needs to have been run immediately prior to the uninstall, so that it can register its set of components with Windows, so that then they will be uninstalled correctly.
I know there are a lot of underlying changes to the Windows 10 security model, so it could simply be objecting to the old MC17 DLL and stopping it from running. If you can run the uninstall in Windows 7 compatibility mode, it may allow the DLL to run.
By the way, I hope you have backed up your MC17 library, and also realise that you may need to install intermediate versions of MC, import the MC17 library, backup the library from the new version, and then install the next version, import, backup, etc. I'm not sure which intermediate versions you would need to install, but if the library format has changed, you need to go through the process to progressively update your library. I'm pretty sure MC21 will not successfully import an MC17 library and convert it all correctly. You should check each version as you go as well, to make sure the conversion worked.
Unless, of course, you are just starting your library from scratch in MC21.