That would be no one's idea of an enjoyable experience. None of us are privy to the modeling the powers that be most likely did while designing this, but I can almost bet that their models did not show enough people purchasing
Genie+ that would result in a 3 hour standby for Space Mountain. Except maybe on Christmas Day or something like that. We also don't know if Disney will cap the number of people who can buy Genie+ in a given day. They already cap park reservations, and Universal caps their Express Pass (in other words, it can sell out). I think it would be wise for them to at least say they are capping it because it will increase the urgency for people to buy it, thus resulting in more purchases because of the fear it will sell out. And I bet they did modeling to account for this too.