Many people still use their original gb (with or without backlit mod, same applies to all handhelds), gameboy colors, gb pockets, gb lights, super gameboy 1 and 2, clones, ect. Making the shell smaller in general would not generate more sales because you would be depriving all of them of a flashcart (unless they used a gameshark on the cart which idk if that would damage the EDGB), or Krikzz would have to make a very limited release of GBA sized EDGB due to demand which would not help cost. As for selling 2 shelled versions, the GB and GBA was not like the Nintendo where there is a famicom and NES pcb due to regions (which is why he makes both versions), so he would probably agree with why fix something if it aint broken (the original does the job) and I personally rather see Krikzz make a GBA everdrive if he is going to make something that size anyways (which he has no intentions to do). Krikzz did mention that if the sales of the EDGB were high enough, he would consider making one with RTC support and make it a ram cart instead of flash, but he probably wouldnt take this into consideration (if he does, retrogate would probably sell a modified GB cart and a GBA cart so the customer can decide what they play on more). If any of the GBA flashcarts (EZ Flash IV) can emulate GB games, that is probably your best bet. PS, this is just my opinion, but GBC multicarts have been made in GBA shells so I could imagine it would be possible.
