This was vanishingly unlikely, especially on a US airline, and both are substantially more difficult to catch.
https://viewfromthewing.com/who-knew-you-can-buy-an-empty-mi...
And I think it's pretty reasonable to be optional, since I wouldn't be worried about sitting next to a family member, for example.
That is, doesn't price discrimination generally allow companies to extract more money overall? And in this case it's contrary to public welfare.
We're not talking about reducing the marginal price on new technology that needed capital investment- we're artificially reducing overall capacity. Airfare already had seating efficiency priced in.
For the math to work, best case, you'd need to reduce capacity of a 737 (3+3 across) by 1/3 (no middle seats). 1/2 for sets of 2 or 4 seats.