Paying to earn FF miles or more FF miles? Before yes, now no. FF programs are now too unreliable to pay for miles. TTN are, for my purposes (but not others, maybe), a waste unless you can redeem the miles immediately.

Pay for a meal? Maybe. It would encourage me to always bring food. The airlines would then have to price the meal so that some food would be unsold and thrown away. Price the meal too high and not many would buy. Price it too low and the cost of accounting and yield management is too great. I have upgraded to get a meal (in essence, buying the meal).