Out of curiosity: what
would you consider being worth getting if you have access to the old material? (Excluding the monster stat blocks and the adventure.)
One of the issues that they have is that the boxed set must be pitched primarily at players who may have heard of Planescape but are otherwise new to it (since the vast majority of current D&D players are new with 5e). So the boxed set basically has to hit the highlights of the setting - Sigil and the Gate towns, the Lady of Pain, the factions. Once they've covered that, there's probably not much scope for new stuff.
(I suppose they could do a full-on reboot of the setting, so that all the lore is new again. But given my reaction to their approach to Spelljammer, I'm not convinced that would be welcome.

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(Also, for the avoidance of doubt: that's a genuine question. There isn't some veiled attack in there.

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