Samsung could surprise but at this point we're not expecting a miracle for owners of devices that are no longer considered flagship models.
While Samsung is expected to release Marshmallow for all flagships released in 2014 or later (don't expect devices like the Galaxy S4 or Note 3 to get Marshmallow), you should expect the company to favor the current flagships. In fact, it already is.
Samsung's first Marshmallow beta didn't arrive for the Galaxy Note 4 or Galaxy Alpha, it arrived for the Galaxy S6 and Galaxy S6 Edge, the two flagship models from 2015. It's clear, at least to us, that the focus is on those two devices and maybe the Galaxy Note 5.
There's always a pecking order and newer devices almost always take precedent over older devices. So while it would be great if the Galaxy S5 and Galaxy Note 4 Marshmallow updates rolled out at the same time as the Galaxy S6 and Galaxy Note 5 Marshmallow updates, we'd be surprised if that happened.
If you own an older device, you'll want to remain patient.