My controversial take:

Linux desktops peaked with KDE 3.5.x. (Gnome is a ghastly abomination and KDE's been going downhill ever since.)

macOS desktops peaked with Snow Leopard (10.6). Everything since then has gone downhill in UX. (Honourable exception: filesystems and backup support. But those are below the desktop level ...)

Windows never had a peak to fall off and has been subterranean for decades. Put it out of its misery and start over from IBM's mid-80s CUI spec.

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