In fact "dumb" TVs also need control circuitry for tuning, input selection, on-screen display, picture settings, etc., so isn't it possible that it works out to

In fact "dumb" TVs also need control circuitry for tuning, input selection, on-screen display, picture settings, etc., so isn't it possible that it works out to be cheaper to roll that control circuitry right into a more sophisticated microcontroller, which is probably produced in bigger scale due to its wider applications? And then, why produce a proprietary "dumb" firmware for the TV when you can just throw Linux on it?