Would it have provided any benefit? CRTs have continuously variable brightness. The bottleneck was having large enough video RAM to even store the higher pixel depth, the processing power to draw it, and the bandwidth to do the output.
Circuitry to convert a multi-bit value to a voltage doesn't seem to have been cost prohibitive. Even cheap devices (the predate the Mac) like the Atari 2600 could do that.
I'm actually not sure why they went 1-bit with the Mac. Maybe they felt having more pixels was so important that going monochrome was a reasonable trade-off.
The original NeXT had 2-bit greyscale (black, white, 2 shades of gray), and that looked pretty nice. It also had lots of pixels and was way more expensive.
There was some code allowing 12 bits RGB display on 8 bits palette screen mode, using very simple temporal dithering (PWM). For example, a demo for old versions of Allegro library. This blog post shows a way to improve on that.
Circuitry to convert a multi-bit value to a voltage doesn't seem to have been cost prohibitive. Even cheap devices (the predate the Mac) like the Atari 2600 could do that.
I'm actually not sure why they went 1-bit with the Mac. Maybe they felt having more pixels was so important that going monochrome was a reasonable trade-off.
The original NeXT had 2-bit greyscale (black, white, 2 shades of gray), and that looked pretty nice. It also had lots of pixels and was way more expensive.