It's not as much a question of a niche as in difference in philosophy. With Maemo, as most OSS, it's about an itch being scratched. With Apple/Android, it's about a developer believing that HIS implementation will the the next best thing and not the other 499 developers who attempted to address the same issue. In more economy-oriented terms, on Maemo development is driven by DEMAND, while in appstore oriented OSes, as awkward it may seem at a first glance, development is driven by SUPPLY.
Also, there's no "app store" where you could make money from applications. So what you could realistically hope for is that those who own a Maemo device may eventually develop/port what they need for themselves and make it available for the public.