I believe the new spice cycle is needed to help humanity survive the Famine Times and The Scattering. Leto II's Golden Path wasn't just about the Atreides line or the BG but for all humanity.
From the end of GEoD:
“How can he live like that?” Siona whispered.
Leto waited for the strength to respond. “Hwi helps me,” he said. “We had something few experience. We were joined in our strengths rather than in our weaknesses.”
“And look what it got you!” Siona sneered.
“Yes, and pray that you get the same,” he husked. “Perhaps the spice will give you time.”
Frank Herbert (1981-01-02). God Emperor of Dune (Kindle Locations 7999-8002). Penguin. Kindle Edition.
He goes on to mention his spice hoard in Sietch Tabr following this which implies other things, but I think one reason his Golden Path was winking in and out earlier during GEoD when he contemplated suicide away from water was because humanity would need the spice for its geriatric properties just as much if not more than the need for Siona and Duncan to have spice to maintain control over the BG or the Guild or for those groups to have a source of spice long after the hoard was gone.