There are some issues with elements of the setting, as
@Parmandur and
@teitan have referenced, though those are probably manageable. But the basic (or advanced, ha ha) problem is that Greyhawk is very tied up with the classic D&D tropes in their original form, and veteran Greyhawk fans are likely to be extra sensitive to any deviation from that. If Wizards changes elements of the setting to include core 5E elements - especially elements that have only become core in the last few years - it's likely to at least greatly disappoint that fanbase, if not turn them off altogether. Keep in mind that setting changes from the
2E era are considered controversial among some Greyhawk fans...
Putting it simply, Wizards would have to be willing to either take the risk of walking back some of the philosophical shifts they've made in recent years, or be willing to take the risk of producing a neo-Greyhawk that pleases neither old nor new fans.
Hence my prediction that Greyhawk is more likely to receive a limited Saltmarsh treatment (as, significantly, it already has), and the Realms will be the featured setting for the "revisit."