I will say however, I expect Snow and Charming to be pivotal at the end of this. With this whole sharing a heart thing, my prediction has always been that the two of them would bite the dust together in a last moment of glory to save Emma and or save baby Neal, which would mirror the pilot.
I think there are several different points here: there’s the narrative question, and the centrality of their stories to OUAT more generally, there’s the branding and name recognition question (OUAT=fairytales=Snowhite) and there’s the contract pragmatics. If the contracts are up, they’d have to redraw a contract for next season, and depending on how they plan to deploy the characters and how much screen time they’ll have, I presume the network could just offer less money. Then it’d be up to Ginny and Josh to decide what they want, and if they’re ready to move on, then that’s that.
Re: narrative. Snow and Charming act more as the armature for other characters — for Regina and Emma mostly. They don’t really have any independent relationships of any significance outside of those two, except occasional group scenes — most of their closer ties (Red for example) got relegated to forgotten character island. This means that they’re already a pretty atomized node in the network. So they might be more useful to the writers as ‘sacrificial’ characters to kill off, sooner rather than later, as a way of moving along the characters they’re more interested in writing (Emma and Regina). They can tie some lose ends — Snowing giving their blessings to Emma and Hook, asking to raise baby Neal as their own — and voila. The end of their journey.
As to how that happens — via the heart share or ridiculous gimmick thought up while drunk the night before — is probably not very high on the list of considerations.