As far as I remember, quidam's rationale is "Ubuntu being the most popular
GNU/Linux distribution, its many users who value their freedom want a free
software only derivative: here is Trisquel".
Trisquel up to version 1.5 was based on Debian and, as far as I understand,
no technical difficulty fundamentally prevents Trisquel from going back to a
Debian base.
Now, in addition to quidam's rationale above, it must be pointed out that
gNewSense, which is now based on Debian (it used to be base on Ubuntu!) seems
to be "reborn". If the objective is the liberation of as many users as
possible, isn't it better to have the two projects differ up to their bases?
