I don't think the set revamp was getting players back (alone), but the first two season themes were bland and pointless and at best just saved some time.
Grandeur seasons actually gave people more power and allowed new build variations that weren't available before, same with LoN season.
Current season again just adds a few minor updates, but the season theme is rather boring after the game altering last two themes. They could just have come up with the season theme "Imaginationless" that adds 15% damage to all players.
And still there is no long time motivation to keep someone busy after one or two weeks. As soon as some healthy "time between updates" margin is passed I will stop once again this season. Some kind of item progression would be nice, so that items dropped by let's say paragon level 800 are generally better than those dropped at 200 (and not just more). This would allow slower but more rewarding progression with less RNG, but I certainly don't think we are going to see any gameplay changes anymore for D3.