[alasdair_]

>it should have been Win+Tab since the beginning

In the beginning, there was no "windows key".

[Eduard]

In the beginning, there was the "super key" https://en.m.wikipedia.org/wiki/Super_key_(keyboard_button).

Its purpose: control the (global) desktop environment (versus e.g. Alt/Ctrl/etc, which control the currently focused/active/local application).

Many "Windows key" shortcuts do desktop environment actions, hence the Windows key is the super key.

[kolinko]

Windows95 introduced the Windows key, before that sure there was no „win” key, but it was not needed much before real multitasking

[quietbritishjim]

Windows 3.1 tasking was technically lacking, but still very useful and still very much needed alt+tab. It needed it even more than win95, in fact, as it had no task bar.