So there is a chance you are wildly under-estimating "the amount of MGP you can reasonably earn each week". The FF 15 crossover event has run a couple times now, with the car costing 200k MGP(and a few other rewards adding the total required to around 300k if you wanted everything). Each time people less familiar with the saucer have come to the boards to either complain about how they could never earn that much in the 4-6 weeks the event was running or to learn how to get that much quickly. Well it turns out that with less then 2 hours a week(im not in game right now to put together the actual minutes required) you can make well over 250k MGP. If you spend your saucer time wisely you can hit 125-150k in roughly 30 mins(between prizes and challenge log payouts).

Also while you are correct that the list of rewards is ever growing, they actually add items to it very slowly. This entire expansion(6.0-current in 6.3) theyve added less then 5 mil worth of rewards, and as you yourself noted 4 mil of those are the one new mount. The entire Shadowbringers cycle was maybe 3.5 mil, and 2 of that was in the sabotender mount.