What exactly is still stopping people from making a Gameboy Color backlit display?

[cvskid]

SP has no headphone jack and infra red port and it messed up the controls for Kirby Tilt in Tumble

You can buy a headphone jack adapter for gba sp but it takes up the spot of the charger.

 

[rad3ds]

There's a places that will install a new LCD screen inside a Game Gear and even a Sega Nomad. Surely these are more niche than what the OP is suggesting?

It's strange that such a mod doesn't exist given that similar aftermarket mods exist for most other handhelds.

The game gear has a rather large fluorescent light bulb and reflector assembly behind the screen. There is a lot more room to work and add custom video converter PCB's to a gamegear. The nomad is so similar to an actual genesis that you're basically patching a tiny television screen into the appropriate spots. Some of the more lazily done nomad mods even have an "AV 1" that pops up in the corner of the screen when the system starts up.