Assuming
all of its dependencies have newer Python 3 versions, and one managed to adapt the Zenoss codebase for changes in those (I'm sure Twisted and Django would not be trivial) along with whatever's necessary to get Zenoss
itself Py3 compliant, there'd still be a necessity to have some kind of Py2.7 environment around, like via containers, for nearly every ZenPack out there. IIRC, even 6.3 still runs on fairly-early Py2.7.
At this stage, it seems tantamount to writing one's own monitoring system from the ground up, using Zenoss as guide.