1) If NYCFC were to sell a player well under the value they establish for them, it sets them up terribly in the future negotiation-wise for all future sales
2) I know you like to pretend any money above what can be used for GAM basically just vanishes, but it doesn't. That is money the club can invest in transfer fees for future DPs, into the academy, into fan relations, or into the local community
3) The GAM money is currently capped at a certain amount and that amount increases each year. Additionally, the roster rules and salary cap have been becoming more and more flexible as time goes on. There's no reason to think the amount of money that can be invested back into the roster won't be more if a sale happens next year compared to this year
4) Loaning a player out can result in GAM as well depending on what the team the player is being loaned to is paying for the loan. We don't know what that is in this case, but it is possible that NYCFC could basically double-dip in GAM on the loan, and then later on the sale.
5) Long term growth for the league. Sure, this could be a bit of a reach, but selling the top striker in the league for $15-20m as opposed to $8m helps build reputation for the league, increasing visibility from within the country and around the world, attracting talent to the league (as well as the club).