That sounds like a decent enough reason, though I still get patches for DSM 5.2 on my DS1010+. Yes, they are security patches for vulnerabilities, but I wouldn't put it past Synology to sneak in a patch disabling "unauthorized" upgrades if they really wanted to do it. In any case, I'm looking forward to the release of DSM 7 so we can see this upgrade process.
I am also wanting to know more about this potential upgrade for my DS1010+. It is a shame that Synology discontinued support even though there is no hardware limitation, only a self-imposed artificial limitation. What is the reason you are waiting for DSM 7.0 to be released?