First, I'd like to say that LMDE2 is tremendously lively for a dead distro. It flat does the job, and probably will well beyond 2020. Of course my needs are not that great, I'm not a gamer so I don't need the latest this-and-that. I need basic business software, I need hardware compatibility (most distros won't run my old Canon LiDE scanners, for instance), and I need to run an old Windows audio editor. Cool Edit 2000 won't run on Windows anymore, but will on LMDE2 under WINE. I need to rip CDs and DVDs. I need to hack Android 'phones and tabs. OK, I WANT to hack droids. I could go on, but you get it: LMDE is a workhorse for me with many long years left in it.
That being said, I was (for some reason) excited when Stretch went stable. I installed 9.1 with XFCE on a poor little underpowered Toshiba netbook-on-steroids, Celeron 1.6GHz w/ 1GB RAM, and it loved it. Apparently I did too, 'cuz I went and bought a 260GB SSD for my old Dell Lat E4300 laptop and installed the same distro on it. (OT point of info: I define the difference between a laptop and a netbook as the built-in optical drive.) This one has a core2duo, faster, with 4GB RAM, and it boots cold to desktip in 20 seconds flat. I've kept up all the updates (if that isn't redundant) (or if it is redundant, I did it anyway), including the kernel just the other day, and it still flies. Systemd-init is going to take some getting-used-to; I can't edit /etc/rc.d/rc.local anymore, it doesn't exist! But so far I've been able to get everything I wanted to working, including samba client. The samba server, you guessed it, is my LMDE2 box with terabytes of storage.
So next upgrade to the big desktop? Still up in the air, but I have years to figure it out. It's six years old now and running strong, never had a bit of trouble with it (knock on wood). Maybe Buster when it goes stable, maybe LMDE3 will have a nice easy upgrade path. For now, I ain't gonna fix it 'cuz it ain't broke.
Final thought: what I will not do is upgrade just because something newer comes along. Newer is not necessarily better; Windows has upgraded to the point where it's useless to me now. Mint main edition doesn't support the scanners I cited above anymore. Nougat ROMs for my Asus TF701t tablet don't support the camera. Etc ad nauseum. So my personal answer to the original thread title is a resounding No, Stretch is not the end for LMDE. I see it as the beginning for LMDE3.