No, not everyone is going to abandon Linux; I'm going to define Linux in this context as the traditional Linux desktop where people run some sort of X11 or Wayland window manager or desktop environment on top of a Linux kernel running on bare hardware. However, it would be interesting to see what impact the popularity of WSL will have on Linux desktop environments such as KDE, GNOME and its derivatives MATE and Cinnamon, and others such as XFCE. For example, macOS has long been popular with those wanting a polished *nix environment with support for commercial software packages and certain types of drivers, to the point that some high-profile developers such as Miguel de Icaza adopted the Mac. Some may argue that the Linux desktop may have evolved differently had macOS not been an option. Even so, the existence of macOS has not led to the extinction of the Linux desktop. Now, what makes WSL different from macOS is every major PC manufacturer ships their PCs with Windows 10 by default, and WSL can be installed easily on any PC running Windows 10. You can run Linux alongside Microsoft Office and Adobe Creative Suite without having to use WINE. The barrier of entry for using WSL is much lower for a lot of people than the barrier of entry for purchasing a Mac.
This leads to some interesting questions. If a significant segment of Linux users abandon FOSS desktops such as KDE and GNOME for the Windows ecosystem, what will this mean for projects such as Firefox and LibreOffice? What will it mean for KDE, GNOME, and other FOSS desktop environments and window managers?
Now, there will be a contingent of Linux users who won't migrate to WSL for a few reasons. There are some Linux users who won't use Windows, whether it's due to certain features such as telemetry and ads or whether it's due to some other aspect of Windows they don't like. There are some Linux users who need to run Linux directly on their hardware and thus WSL wouldn't fit their needs. There are also Linux users who care about using a stack of software that is fully free and open source and thus wouldn't switch to SWL.
But will there be enough remaining users in order to justify further development of the FOSS Linux desktop in the eyes of major projects like LibreOffice and Firefox? We will see.