Flight watch is provided on HF.
And is also an "on request" service on the ATC area VHF freqs, but you may have to wait.
that IFR and VFR in the same airspace be on different frequencies? IFR with ATC, and VFR with FlightWatch?
Err... No requirement for VFR to be on any specific frequency - just keep your eyes outside in E and G. Don't forget the gliders who have their own VHF and have for many years.... Even when there was FS. Their greatest risk is colliding with another Glider! We now have CTAFs, MULTICOM and BROADCAST AREAs as well. Good airmanship should have you using the best one for the circumstances
FS in its day was outstanding, but we have moved on and the facilities have now been pulled or transferred to ATC, who can provide a similar service "on request". As opposed to the old days, not many VFRs use it now.
Much as it was nice to have FS to be available to hold your hand when the going got tough, I would suggest that with the large reduction of VFR traffic over the last decade or so, the cost-benefit would not be attractive $wise.