Plus deepslate bricks and blackstone bricks have nearly the same colour, not quite, but almost and unless you have 20/20 vision or even above average vision it can be hard to tell the difference between the two, even with good vision it still takes a well lit room to spot the difference.
But one thing that bothered me and still does about deepslate is it has the same blast resistance as stone does, and yet it takes much longer to clear this material away to uncover ores you need. If it were made immune to a non charged Creeper explosion, it would have been useful.
I would agree with encouraging players to do caving and seeking out dungeons and abandoned mineshafts for interesting loot so strip mining is no longer necessary to get what you need but the Caves and Cliffs update doesn't do this, we got the worst of both worlds and now both are discouraged, forcing more dependency on things like mob farms and Villager trades.
It sticks out when you build in an Ancient City, plus deepslate has a tile variant that blackstone completely lacks, as well as a wildly different chiseled variant. I'm very avid deepslate builder, I'm not going to let that material go.
I mine a lot, and didn't find myself really in need to strip-mine for minerals too much; Lapis and redstone alike have the supply well exceed the demand, gold can be and is acquired from zombified piglin grinders if someone wants to run a mass bartering setup and isn't needed in large quantities elsewhere, coal and emeralds are unaffected by deepslate.
With diamonds, fair enough, they are hard to get, but villager blacksmiths more than compensate for that.
And lastly, iron, which I found the easiest to acquire en-masse not by strip mining but clearing out iron-tuff veins, and whose strip-mining is done above deepslate layer anyway, at Y=15.
The one thing I would like to see with deepslate is increasing its blast resistance to resist uncharged creepers; it may not matter much in Ancient Cities but gives big incentive to build elsewhere in Ancient City style.