So yes, maybe some people will stick to older Tycho versions, but that's their issue,
Exactly! The old versions of Tycho are not going away. The old versions of Eclipse are not going away. If people have hard requirements on Java 8 support, they can continue using these versions since they will be available in perpetuity and projects, as far as I know, are not obligated to upgrade.
I don't understand the attitude that says, "I'm stuck in the past, so everyone else should be stuck in the past, too." Let those who want to move into the future do so unhindered :-)