Yes, dumb users are unable to exercise choices meaningfully, especially if they are unaware they even have a choice. But that doesn't mean that choice doesn't exist, and can't be exercised by even somewhat competent users. Googling "ubuntu change desktop environment" returns easily followed instructions for doing so via the GUI, so the only requirements are knowing how to use google, and what a desktop environment is (if they don't know the latter, they could easily find out by posting on the forum, or asking whichever person told them to try Ubuntu in the first place).
DE-specific releases don't exist because they're the only way of installing a DE, they exist because most people want to have a DE out of the box, but disagree which one to have.
Gnome 2 was released in 2002, and supported at least until the release of Gnome 3 in 2011 (9 years). In contrast, the WinXP was current for 6 years, and the Vista interface for 5 years until Win8. KDE has had a major release every 6 years, comparable to Windows. Xfce has been on version 4 for 12 years. Additionally, KDE4 is still supported, as is Gnome 2 (under the MATE project).
So by the numbers, the popular Linux DEs are at least as stable as Windows in terms of UI.
Now, if you want to argue that Linux is more fragmented in terms of UI because of this, that's a different discussion, but doing an apples to apples comparison shows that it is as stable/disruptive as Windows in the worst case (KDE), and significantly more stable in the other cases (Gnome and Xfce).