While there is RNG, there's also baseline directly tied to difficulty, same as it's always been, and several avenues available to you for free, guaranteed baseline(+) gear in the form of follower missions, weekly mythic + chests, and timewalking chests when those pop up (I'm unsure if the pvp / pet battle weeklies drop gear)
I think the bottom line is that if you are actively playing, you will see progress up to the given difficulty that you're participating in on a fairly predictable scale.
The underlined is key here. I do mythic + content, normal and some heroic raiding, and my equipped ilvl reflects that pretty accurately (887, probably shifted down due to an 840 relic that I haven't gotten a good upgrade for yet). Drops that are upgrades for me at this point seem to be semi-rare, but again, considering the content I'm doing, that isn't a surprise.
In the older versions of how gear in this game worked, I'd be at the same comparative gear level, and simply have no reason to do normal mode raiding at all, for example.