I kind of agree with the OP. Suppose you were flying route AAA-BBB and both AA and UA fly the route. UA has 10 rows of E+ and 10 rows of E-. AA has 22 rows of E. Both flights are about 60% full.

On the AA flight, most people will get a free seat next to them (which makes the flight 10 times more comfortable - especially on longer flights)

On the UA flight, the Y- cabin will be packed but the Y+ cabin will be empty almost (assuming fewer Elites than non elites - kind of like the OP described.)

So because of the existance of Y+, Y- passengers are getting slightly less comfort. However as previously stated - UA wouldn't do this if it wasn't revenue generating... so more power to them ^

Cheers