I was a bit confused about how things would work before this update. The way I see it, current character is good for campaign, clearing fog of war, discovering Altars of Lilith. Aside from that, maybe hitting max level and gearing up completely if you really want to play this game beyond seasonal characters.
That's pretty much Eternal. There's no real end-game beyond that, unless I'm wrong. If they raise the level cap, ntroduce better gear, or add more content on Eternal, then there will be a reason to return to your main character.
Seasons introduce new mechanics on Seasonal servers and unlock cosmetics for all characters.
I don't get the comparisions to MMOs. Those games generally have a proper end-game, continue to expand, and encourage you to continue playing the same character. This game has never felt like an MMO to me, except maybe when I ran into a world boss one or two times and people gathered to kill it.