I want to agree with you, I really do. You have some great arguments from a casual to semi-casual perspective, but remakes/remasters, like their OG counterparts, are held up by those that invest a LOT of time into the game. Casual players come and go, while semi-casual players face intense burnout (I would personally consider myself semi-casual to slightly more than that, for reference), but to be frank, casual players did not keep this game going.
One additional thing worth considering is how ladder exclusives impact new and returning players. When a new ladder comes around, new/returning people can not only have a more level playing field, but the chance to obtain exclusive items that make their casual stint more rewarding. Items are worth SIGNIFICANTLY less in non-ladder, so if everything is available there, starting there from the get-go would be an uphill battle.
The last thing I want to say in favor of ladder exclusives is the economic impact it has had and will continue to have on the game. Ladder exclusives keep the market fluctuating over the long run. Every four months, the economy is restimulated by harder to obtain items and everything in the market adjusts for that. If everything is always available, the excess supply continuously drives the market downwards and doesn’t allow for demand to catch up. That isn’t healthy for the game long term.
To reiterate, I don’t think ladder exclusives are the best option. I think that Blizzard could devote development time into continuously creating new content and/or constant balance adjustments to constantly stimulate the game, but that costs a lot of money and time. Ladder exclusives aren’t as good but adding a couple runewords to ladder takes orders of magnitude less investment from a development and business perspective. It is the “bang for the buck” method.
Consider Path of Exile as an example. If that game had all Standard all the time with occasional new items, they would have flopped HARD within a year or two. They have both additional regular content and exclusive “ladder/league” items and they invest significantly more money into PoE than Blizzard will D2R.