I bet this will go well
It wont end well, we all know this.
BUT to be fair this seems like the least bad way of doing it. They're trying to go cross-platform as much as they can without doing "lowest common denominator" (think not QT, which renders its own crap, and wxWidgets which gives a standard API to native) stuff. This is good.
Take the "universal apps" - yes the name is a lie, but there's logic here. For apps which are simply front-ends this is a pretty good way of doing it, it becomes a bit weird when that program will use some resources (phone compared to desktop) but it's a good attempt.
I'll never use these tools (Linux! (Mint MATE)) but I can see that they're trying, it may not be great but it does seem they deserve credit for the least bad way.
Also regarding security, I would hope that if they find a problem that they release/push (whatever) packages containing JUST the security fix to all versions, that way version n will behave like version n just with the fix.
This means maintaining 4 versions per year which isn't nice but it's not a huge task, especially not if the fixes are trivial.
Just a thought.