I think Reason has become niche by accident not design, and it's a commercial point not a point about how the product operates or what it does.
Reason is a pretty good product that IMO easily compares with the best in the market in terms of plugin instruments and (for all its faults) isn't the worse DAW in the market either (and the DAW could get better over time if RS decide to fix the list of things that bug people).
The problem is that, over time, Reason has fallen behind the other major DAWs in terms of adoption (and also behind the major plugin firms like NI). Several factors behind that. Some of them are down to management obduracy (eg refusing point blank to consider adding VSTs, for years). Some of them are down to changes in user patterns and behaviours (old guys like me grew up with hardware and cables and love the rack for that reason.... but skeuomorphic design isn't common elsewhere in the digital world now). And some of them are just down to commercial mis-steps (e.g. Ableton have pursued a super aggressive software bundling strategy for years, offering free Live Lite licences with so many different hardware manufacturers and a clear and simple upgrade path.... where were Props/RS all that time?).
So for "niche" read: small user base. But Reason does not, in itself, have to be a niche product. It didn't feel niche back in the day when Reason often made the cover story in music production magazines and a lot of producers I knew used it daily. And - I think - it can stop being 'niche' in the future if RS/Verdane turn things round.