Well, that's precisely why I said "PROBABLY the biggest" because different people might define it differently. Of course, things like number of clients, financial backing, etc. matter but I don't think it's unreasonable to say that profitability is one of the most important factors indicating the potential longevity of a business, & in that regard, I think Zerodha has been doing really well since inception (which COULD change in the future but of course, future is inherently uncertain but so far as probabilities go, FOR NOW, they might be on Zerodha's side).
Right, I was thinking about RKGlobal too but for some reason, they don't always get a mention in the list of discount-brokers. I've thought about opening an account with them in the past but I was never sure about what their service would be like, I guess I've been more comfortable with opening an account with brokers on Traderji because it seems like a place where traders, as a community, can try to hold brokers accountable, & where we might have a greater chance of being heard but perhaps, I need to broaden my horizons (which I sort of have tried to do recently, by initiating account-opening procedure with Interactive Brokers).