Personally I have no doubt that Linux Mint will eventually produce LMDE 3 based on Stretch.
However my feeling is that Mate is not very important to Mint, perhaps unimportant.
The version of Mate that Mint produces is unique. They have created a version that is very much like Cinnamon in layout, and it is distasteful to me. I have been using Gnome 2 and Mate for a long time and I find the Traditional layout and function as ideal for multi-tasking and office productivity... less keyboarding and less clicks to execute simple repetitive tasks.
No big deal, I can rearrange panels, the Menu Bar, and such while the initial update is running.
but now function is lacking:
I constantly drag and drop files and folders to the panel for quick access during my workday, and delete them when done... and that basic function has been lost/deleted. I believe it is on the todo list, but maybe not.
I do understand that Mint is aimed at making everything very windows like for the new converts, I am fine with that, but I need to work.
STABILITY is highly important to me. Set it and Forget it. Just use the darn computer and ignore the OS... just accept the updates. Did we loose that goal of stability?
All of my LMDE Mate installs are slowly going away, either because of loss of function or updates resulting in black screens.
I want my LMDE back!
Sorry for the rant