I know twinks are affected by this, but it's basically fixing itself in a few weeks when the majority of players don't need that anymore because it's a one time thing.
I leave dungeons early too btw. Not only for legendaries though.
I can check the loot-table, unless I'm running with people I know, I'll leave early if I know that the last 2 bosses can't drop an upgrade.
Since titanforge isn't a thing anymore, it's basically pointless as there is no reward in playing any further. Bosses can only drop 1 specific item most of the time. For example bracers.