without new cartridge games I feel like things can easily shift away from it again.
I do not know the Game Boy cartridge market well, but I know some people still upload their own created games to cartridges and sell them; for example, I know of someone (who took part to Game Boy Showdown, actually) who noticed there is no official Pong for the Game Boy and decided to create and sell one, and apparently, copies sold quickly. I also stumbled very recently upon a similar account from a few years ago for a Breakout clone on Gameboy Development Forum.
This may be a niche, but there seems to be a lively core; I’m sure emulation helps. If you look at the reactions on the game page for the first-ranked game from GBJam 9 (Necrow Co.), you will notice someone suggested adapting it as a genuine Game Boy ROM, and people were willing to buy it if it became a longer, full-blown game, so if some talented folks out there make something new, I suspect success could come knocking. (I am still learning, but interested; we will see!) I also have close family that dealt for some time in buying and sprucing up dysfunctional portable consoles, and there is quite some demand. I am not sure how successful the Thumby is, but this looks like another sign (it has even less colours and pixels!). And heck, the whole retro revival and demand is also totally in line with the Game Boy spirit; there is a lot of (sweet) nostalgia going on, as someone who played great games on it, I would love to see ambitious Game Boy games come out. This is actually one of the reasons that sparked my interest for trying out Game Boy programming. (As a coincidence, looking again today for editions of Game Boy Showdown through Lilo, I noticed two video results featuring the glorious 1994 Donkey Kong, on the thumbnail of ‘10 Game Boy Games that still ROCK in 2019!’ and a remix of its Showdown at the Tower track.)
I also guess GB Studio might help bring new people to it too (and the logical progression if you want to pull more off is to then learn Game Boy coding); version 3.1.0 was apparently a rather big update, and is from only last 11th September. I also noticed through its name that GBDK-2020 is rather recent, and found out about various rather recent tutorials on it (either webpages or videos); I think there is an interest, and since programming for Game Boy seems to be a not-so-common skill, if people are willing to become better at it, they will have a sought-after craft. (Only using C now, but nerdy me cannot wait to delve into ASM/assembly! XD Learned some at university, loved it.) I realized it combines at the same time assembly/C programming, pixel art, and tracker music composing; this is like the ultimate Nerd Skill Machine. XD
Very curious to see what game jam dedicated to the Game Boy have to offer. If there is not enough on real Game Boy games, maybe I will create one, someday. :) Just musing, for the moment!