In a head-to-head sense, Ngannou is already an all-time great. Are we really gonna pretend he wouldn't have a good chance against every HW ever just based on his power, strength and chin? Especially when he doesn't just rush in like a madman like he did against Stipe.
And current DC is better than younger Strikeforce DC. Cormier himself has gone on record by saying that going to LHW and fighting Jones made him a much better fighter by forcing him to evolve.
Ngannou, Stipe AND DC are all amazing fighters.