Supporting this argument: They've worked with ABS since 1963, Lego molds have a tolerance of less than two micrometers (they don't connect if they are out of whack), and they've produced 400 billion blocks over five decades.
PLUS they are super-durable, non-toxic, and have these cool colors:
(Plus other colors, I think, including metallic ones.)
And the larger boxes of blocks cost just a bit more than a penny apiece, so you know they could have a healthy margin with keycaps.
Just $75.99 on Amazon.
(Seriously, they would be awesome. I still prefer PBT, but some of those translucent Lego bricks would be perfect for backlit keyboards.)