Once costs for a specific flight are covered by revenue for that flight it's all an incremental game. This incremental game impacts RASM-CASM (eg, profit) but is a different measure. A last minute flyer paying $73 for something with an incremental cost of $72 garners $1 profit. Profit is profit.

However, if 3 other passengers who would otherwise pay $173 know that the $73 fare will be available they'll wait and the airline will loose $297.

Note that what gets hidden with CASM/RASM is that CASM is different for every flight. There are fixed costs that are about the same for a 100 mile flight as they are for a 1,000 mile flight. A company with low CASM and high RASM but does mostly short flights could have much lower ROI than one with a mid CASM and mid RASM but longer flights or more efficient ground ops / corp overhead. In the end, at a macro level, it's ROI/ROCE.