The issue is you are only basing it on Triple Triad, there are also the variables of cactpot, GATEs, selling duplicates of triple triad cards, when you just base it on "winning" in triple triad then of course it's going to take a long time to obtain. However when you put the other variables into account it is possible for it to take less time and if you get lucky with cactpot even less time.

A quick tip, the Bahamut card sell for 600 when you no longer need it so if it's possible to get any other cards of that rarity range as duplicates you can also try it with them.As far as I know the price of other cards are 5, 10, 20 and 50. I'm not sure how much other cards go for but seeing that Bahamut does go for 600, winning a second Lightning Card might also impose an additional increase in MGP gains.