Steven P. McNicoll writes:

You're drifting too far off the issue here. What are the advantages
of IFR to VFR-on-top then cancel over IFR to VFR then cancel?

VFR-on-top is still flight under IFR. Cancelling IFR and switching to VFR is
not. If you anticipate that some remaining portion of the flight will require
IFR, it makes sense to remain under IFR with VFR-on-top, rather than cancel
it.