The size of the user base (which has been pretty flat the past six months at around 15k trusted pubkeys daily, as per the screenshot below). There are few apps that can be financially sustainable at this scale, and it forces tough choices.

You can take the risk of developing for a future with 100k, 500k, 1m or more daily active users, but you'll need a runway of several years likely.

Or you can develop something niche that might be sustainable with a modest-size userbase, one of the size that nostr could hit some time in 2026 if things return to growth after the summer. But that would be a user base with a very focussed need, and that may lead the app to get stuck.

Or you can build something that uses nostr but targets users off nostr altogether—nostr is just something slipped in, some part of the stack. Again though, that approach comes with its own risks and tradeoffs.