Yeah Demon’s Souls does have something like the Megaman structure, which is a comparison I haven’t thought of before.
And one way of thinking about the grass/Estus systems is that From aimed to reinvent the “lives” system of NES games. “lives” were a rightly abandoned mechanic by 2009: they’re opaque, rigid, and the way they carry over gamelong (whether in the direction of abundance or scarcity) is imbalanced. But From also realized something of value was lost with them.
In many other ways though, Demon’s is absolutely nothing like NES games. NES games are defined partly by the extreme simplicity of their controller and general absence of RPG upgrade elements (Zelda II excepted – BTW another good game to compare specifically with Demon’s, particularly the bleakness of its world).