> It seems a little like cheating to put modern microcontrollers inside the cartridge that do all the heavy lifting.
As a sometimes hobbyist homebrewer, instinctively I am inclined to agree. But I'll argue the counter-case.
The Game Boy (and consoles like the NES and SNES) were intended to be expanded with additional hardware in the cartridge, from the beginning in anticipation of new technology. There were commercial GB games in Japan that included an infrared transceiver, for example. Same with vibrators and battery-backed save RAM and so on. While extra cartridge hardware was never more than marginal on the Game Boy, it was fairly prominent on the NES/Famicom and especially the SNES.
And it was used to make full 3D games for the SNES. Much of Star Fox is drawn with 3D polygons, and would have been impossible without a co-processor to do the 3D rendering. So it includes the Super FX chip on-board, which is a fast coprocessor with vector facilities. In some Super FX games, the console's baseline hardware was basically relegated to being a glorified framebuffer displaying the coprocessor's rendered output.
And in some cases, no amount of extra hardware will circumvent the limitations of the hardware, which makes it a legitimate programming and game design challenge, IMO. You just can't display a full-screen bitmap on a Game Boy without tricks. There simply isn't enough onboard video RAM to hold a distinct 8x8 tile for every tile on screen. No coprocessor will work around that. It might give you an infinite source of tiles rendering a 3D scene for example, but you still have to shuffle them in and out of VRAM, which is a major bottleneck for complex Game Boy designs aiming for full screen and full framerate effects.