Clem was interviewed on MintCast here:
http://www.mintcast.org/2011/01/episode-50-interview-with-clem-lefebvre/. They did discuss what would happen if it became too much of a challenge to base off of Ubuntu (if they made it Unity-only, for example), and Clem says that at that time (which was January 17th, 2011) that they were confident that they could move to Debian. I think LMDE suffers right now because it doesn't have a large userbase (about 13% of Mint's overall userbase according to Clem) and it therefore doesn't get the amount of work it needs to be a replacement for the main edition, and because it isn't a replacement for the main edition (for many people) people don't use it. If Mint committed to switching fully, I'm confident that they could make it work well and iron out all of the kinks. SolusOS, which is a great Debian Stable based distro (although some people in their community aren't big fans of Mint

), one part time developer who was the person who originally worked on a lot of parts of LMDE. They have other contributors, but he does most of the work, and they have made a lot of improvements on Debian with those small resources. Mint doesn't exactly have a lot of resources, but we have two full time devs (clem and fredg) as well as a number of contributors and good name recognition, so it would be feasible to switch and improve.
The Mint 15 roadmap is here:
https://github.com/linuxmint/Roadmap, and in this roadmap at the bottom it says:
R&D on "from scratch" package base and investigation on pros and cons of dpkg compared to other packaging systems (multi-version installation, static/dynamic support, snapshots, delta, update reversals etc..)
This suggests that Clem and team are at least looking at other bases/package managers. If there is enough R&D they could be fairly prepared for Ubuntu not being a feasible base (or if it dies).
Dell XPS 15 l502x - Debian Testing 64-bit NetInst Xfce, SolydX 64-bit Debian Testing, SolydK 64-bit SolydXK Testing
Old Gateway Pentium 4 Desktop - Arch Linux 64-bit Xfce and SolydX 32-bit Sid