No matter how profitable building good planes is, building slightly worse planes that are bought in equal numbers is more profitable. The profit maximizing move is to lower quality until sales suffer more than profitability is increased. And if you have a strong reputation, and feedback cycles on product quality are long, lower quality might take a long time to actually be noticeable in sales.

Boeing profits and sales had been on a steady upwards trajectory for 20 years until 2019:

https://www.helgilibrary.com/charts/boeing-profits-sales-qua...