Well, problem number one, who is going to pay for and administer an ICAO seniority system? Those pilots with little or no seniority may offer to pay but those with considerable seniority won't pay and there is no way you will get an international law agreed that says pilots must pay.
It has all been said already, seniority isn't a perfect system but in our industry/profession it is the only system that works and can be seen to be fair.