Yeah that’s the sad thing with Blizzard, they often make decisions which make sense on paper, but which in practice serve to make their games less fun and less successful, and there’s a decent chance Delves will meet that fate.
Right now, they’re extremely popular, and whilst they offer a good reward, as people progress through M+ and Heroic raiding (let alone Mythic raiding), that reward is going to gradually become less and less relevant for anyone but people primarily playing Delves. That’ll be true in S2 as well, even if they fully updated T8 to give the S2 equivalent gear, Delves would have maybe 2 or so weeks of general relevance, if that.
But Blizzard tend to overreact and make decisions based on last season’s data, and often data from very early in that season.
If they do nerf Delves, they’re going to drive away an awful lot of players and drop an awful lot of MAUs.