Pity. They intend to make Sableye into another one of those dubious Trick Room set-ups, only they forgot what made the base form viable. Sableye didn't need a defense or SpA boost, it badly needs a SpD one. Spiritomb already exists and has +105 BST on Sableye (Megas give +100 BST, not entirely a false equivalency) and nobody uses NP on it. Obviously, the BST is going to be different and Spiritomb doesn't get Recover or nice abilities, but we have to be mindful of possible stat distributions - I would be skeptical even if it did have usable, balanced defenses. Mawile had to get Huge Power to be useful coming from those shit stats. Remember that Sableye's durability is somewhat artificial thanks to priority WoW and Recover, so it's not guaranteed that the Mega forme will hold up to what the base form can.
Yes, you can delay Mega Evolving, but that assumes the Mega forme is worth using period. We have to wait for BST to make the final judgment but I'm not sure how well it can abuse Magic Bounce and its lovely utility movepool, or some contrived special attacking set that people keep spouting about with abysmal speed. It never needed attacking stats because Foul Play or Night Shade suffice for damage. There are plenty of other good Ghost or Dark types in the special attacking department (Gengar, Chandelure, Houndoom, etcetc), not sure what niche Sableye would fulfill that the others don't.
However, after all of that shittalking, this might find a home on stall teams. As Karxrida said, the other Magic Bounce users are lacking in sustain and status that Sableye nicely patches up. Stall teams might like the unique typing in conjunction with Magic Bounce, sustain, and WoW; it fits the archetype and offers something fresh and new to those teams. I don't play stall, so I can't really elaborate, but some stall player might want to weigh in on this. It's nice at least to have another defense-oriented Mega to run other than Venusaur.