With that GRUB configuration, the latest kernel should boot by default. I
can take a look at your /boot/grub/grub.cfg if you attach it to a post but I
do not promise anything. It may be that libreboot's things interfere.
Older kernels do not substitute newer kernels because if, for some reason,
the newer kernel does not boot (or just does not drive some of your hardware
as well as the older kernel), you are happy to have another kernel to select
in GRUB's menu.
Talking about the GRUB menu, have you tried to boot the newer kernel from
"Advanced options for Trisquel" (or something like that). You know about the
useless GRUB password and how to learn it or, better, get rid of it, don't
you?
