why have a front shifter, cable, and derailleur, and multiple chain rings when they are not required, you can have a 12 speed with only a rear derailleur
36 speeds baby!
(Just kidding, your point is well taken.)
With that many gears in the cassette with only a single chain ring, would cross chaining become an issue when in the lowest or highest gear?
No more than on an 8-speed cassette. The stack height is the same, the gears are just closer together on the larger cassettes.
Why have a derailleur at all? Just get onboard the Rohloff e14 + Gates Carbon Drive train.
I mean chains are a relic of the past. The future is carbon belt drives.
(See, I can be snobby, too).
They tend to trap mud and dirt easily. Also, simplicity is desirable. Less to wear out or go wrong.
Conceptually, one less derailleur is simpler. However, the design and manufacturing of 12-speed full-range rear shifting systems is pretty darn complicated. I run 2x9 on my main MTB, and love the simplicity of the parts.