... whether Linux Mint may eventually lean more heavily on its Debian roots rather than its traditional Ubuntu base.
I'll second the click-bait concerns mentioned already in other comments. "In trouble" is too charged a term. That said, the last time I tried LMDE, I scurried back to the Mint based on Ubuntu - LMDE was just less refined and needed more intervention to get what I wanted.
I started out my Linux journey on Debian, but found that it became harder to maintain, and that even the "stable" version needed more effort than I wanted to put in to get what I wanted. So I went to Ubuntu, but jumped ship when they started pushing UIs and desktop interfaces that pissed me off. I tried Mint, and it turned out to be the nearly-perfect version for me.
What that means for me is that if Mint starts switching to LMDE exclusively, I may end up looking for another distro. So if there are a lot of other users like me - and I really don't know if that's the case - then it's just possible that Mint WILL be "in trouble" in the future if it drops its Ubuntu-based releases.
My "other mind" doesn't trust Ubuntu as a company, and welcomes the prospect of a distro that's not constantly fighting off the Snapification that Ubuntu is pushing. I have nothing against self-contained executables, but my experience of Snaps has not been good, and I much prefer AppImages - they "just work". I understand the objections around both memory bloat and storage bloat; but AppImages allow me easy access to applications - and versions of applications - that I can't get through repos without a lot of fiddling and risk of breakage.
I'm not too worried about this development just yet, but I think it's about time I gave LMDE another try. If I don't mind it too much, I may start migrating before it becomes necessary.