What about MSFS. Cant i let MSFS Scenery on the Drive and just install AddonManager to connect them on the new PC?
Yes, of course you can, provided you understand the place where the scenery is REALLY installed into, IS the Addon Manager\MSFS folder, the Community folder only contains a symbolic link to it so, the real files are in the Addon Manager folder and the scenery installer creates a link to it during the installation.
So, provided you "let the MSFS scenery on the drive", which in this case really means "Don't remove the Addon Manager folder", the only thing you need in case you LOST THE INSTALLER, is recreate the symbolic link in the Community, which you can either do manually, or by using any of the available utilities.
That is just to make a quick reinstall, provided all your files are still there from the previous install. However, it's best if you do the "proper" route, that is getting your activation key and using the installer because, without a reactivation on the new system, you won't have access to updates, since the license is checked by the updater.