Having flown Sri Lankan a bit this year in J, I see several fundamental problems with them as more than just a small player for O&D traffic for Sri Lanka: 1) Colombo airport is awful; there are no respectable overnight hotels near CMB; the immigration is a mess; the security is outrageously inefficient; the layout is poor; the lounges are poor and; there are limited facilities for folks post-security; 2) Colombo's location is poor--out of the way vs. the ME3 and a long, long flight from Europe (and not currently directly accessible via the US); 3) while the in-flight hard and soft product is great in J, Sri Lankan op-ups/cheap upgrades to an egregious extent, and if you've paid full fare (which I did on each of my flights), you see an empty cabin at booking and then board to find a totally full cabin--often with lots of kids and possibly employees/friends.
They're much better off going the niche route, with better marketing on flights to HKG, LHR, and CDG, forgoing most other international routings, and building a better infrastructure for offering connections to points within Sri Lanka. I would've flown domestic if I knew how slow road connections within SL are and the airline had better positioned itself to offer those flights.