There are lots of ways to play Game Boy games well enough from GBA to GBP to SGB, so I don’t see a crazy rush of demand as everyone suddenly goes to buy GB games they could never play.
I was worried about Sega import prices with the Mega Sg, but how long has that thing been out and they still don’t have the cart adapters? If/when they make an SG-1000 or whatever adapter I think the hype will have died down enough to have a smaller impact than if it launched with one.
I also think because it has an SD card slot, tons of people are just going to load it up with ROMs. Many people don’t care about playing carts they just see the letters FPGA and want that thing.