My understanding is the Mint team - not wanting to keep all their eggs in one basket - maintains LMDE because they want a Debian-based alternative for the OS, just in case Canonical or Ubuntu goes belly-up. So if that happens, Mint and all the people who depend on the OS won't be stranded.
In which case it would make more sense to base yourself on Debian and find a way to retrofit the stuff you like from Ubuntu, wouldn't it? Which brings me back to the same concern, it is not a clear choice when 2 distro's are to many users, very similar. As to the DE, my main interest is MATE, but any distro should allow me to choose. Which is why Debian 10 won over Debian 9, newer MATE. In addition, every upgrade should move me forward. With LMDE 2 and Jessie backports, my kernel was at 4.09, with Debian 9, it is, 4.09.
My hardware required something that came with 4.10+ (I forget the details), so in order to not have to go straight to Stretch backports, it seemed to make more sense jumping forward more.
Then there's the systemd discussion. I have no real preference, but work on the idea that the larger the user base, the better the distro will function, so if Debian and Ubuntu move to systemd, it makes sense to follow that. Unless you're prepared to cope with the fall-out yourself, which I am not. My LMDE 2 was already converted to systemd, and personally I thought it improved startup speed. And yes, it does take some getting used to and I also have read up on why some don't like it. My motivation is different.
It's all about choice, which is good. And so far it seems that LMDE 3's main USP is that you get an up-to-date Cinnamon on Debian 9. O, and a better login window (IMO) with MDM. I did manage to install LMDE's MDM on Debian 10 (a VM), but as it conflicts with another package with the same acronym, for my main system, I stuck with LightDM. LMDE 2 is just nicer in some places, however it is not that much nicer that I wish to trade it for remaining on a older software set that makes some of my requirements a little tricky to meet. But it still feels like the mind says to leave, but my heart is struggling. Weird this sentimentality....
Perhaps an idea for LMDE 3 would be to create a repository that augments Debian, rather than replace it, as a place to offer the latest MATE, Cinnamon, MDM, and some of the other bits and only go for a full repo if the Ubuntu route fails. The reduction in maintenance work might worth such an approach. I don't know.