The "closed-source restrictions" are on the SDK itself, not applications written using it. At risk of being a bit flip, there's a pretty long history of people developing applications that run against proprietary SDKs. The "niche hobby platform" might prove to be more of an issue, but I don't think anyone developing for -- or for that matter, buying -- the Playdate is doing so with the expectation that it's going to be the next Nintendo Switch.