Commercial SBCs are quite featureful, but their feature set is extremely static, and often depends on vendor consulting and expertise to be approachable.
Nobody claimed the open-source solutions are simple to support or don't require expertise. They're not free, either, as everyone well knows.
In my view, the real value of something like Kamailio - where it's not merely a money-saving measure but can achieve truly spectacular multiplier effects in technological leverage - is in its integration paths (MI and sercmd/binrpc), HTTP client/server, database connectors, integrated servers, etc. The ability to develop new kinds of applications and services that rely on making disparate components talk to and interoperate with each other in novel ways, as well as to make better, more economical use of existing technology through those same kinds of facilities, is where the real power truly lies. Some commercial products powered by proprietary stacks have APIs too, but they're light years behind the open-source world in this regard; they are far too bureaucratic and inflexible.
This is where our customers typically see massive payback of using something like Kamailio, aside from the any savings on licensing costs (both of the core network element and potential dependencies).