Yeah but GP membership does give a discount on purchases of GP games. Handy if you get hooked on a game that ends up leaving the service...just make sure to check the “Leaving soon” filter a few times a month.
Also to sub numbers, you have to realize that folks didn’t just pay $1 for this but in many cases added a year or more of standard live sub along with it. They also are likely converting many that otherwise don’t sub year round for either gold or GP because the deal is so insane. They win in this case as their monthly attrition numbers will likely go down. Sure, they technically aren’t getting $15/mo from majority of GPU users...probably closer to the $7-9 mark but if the overall sub numbers go up, they win either way. Another point is that this has been huge PR for Xbox and Gamepass as a service which is another angle that makes running these types of promotions worth it.