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The cautious approach for LMDE5 users: If your system is working fine and there are no especially must-have features in LMDE6, there is almost certainly no rush to upgrade. Take your time.
Make backups. Test backups. Play games. Work. Do things entirely unrelated to the distro.
You could even almost (aaalmost) completely forget about LMDE6 (but do keep an eye on the LM blog).
The Mint team haven’t announced an EOL date for LMDE5 yet, but if past dates are anything to go by, it’ll be at least 18 months before they pull the plug. Even then, LTS updates might still filter through from Debian proper.
[How many people will actually see this message and how many it actually applies to out of them might well include me and literally one other guy somewhere else on the planet, but if you’re that one guy, breathe friend. No rush.]
To some extent I think they’re thinking of people who are in the Windows/Mac situation of wanting a stable OS that doesn’t require getting hands dirty (so to speak) every 5 minutes to do basic things, and who generally call in a relative or friend who knows what they’re doing (and is almost certainly the person who installed Mint in the first place) when things really need changing.
There’s never more than two LMDEs active at any one time, so while they are giving themselves a little extra work, they’re also managing the main Ubuntu-based Mint derivatives at the same time so they’re bound to have some kind of streamlining at their side.
As for 5-to-6 upgrades, they’ve provided an official tool that will work for most people and will require very little admin user interaction once it’s off and running. A sensible sysadmin would like to have a backup anyway, just in case.
My initial comment was aimed at the odd rare case like myself who isn’t always up for sysadmin work (it’s why I’m on Mint after all), or doesn’t have the time. There’s no immediate rush to use that official tool. Take your time. Make your backups, etc.
If you want bleeding-edge rolling updates, Mint is not the distro for you (though LMDE is a little closer to that than regular Mint).
Do they keep up with security updates and patches, though? Yes. Very much so.