But when are you going to get it, or any of the others? The game will only let you store 20M "free" credits at any one time, so it won't even let you save up a whole bunch of credits so you've got enough ready for when they all come around again, and are again limited to 6 days to buy them.
If you get up to 20M credits in another three weeks you have to spend them, otherwise you can't continue earning any credits. So if the McLaren doesn't come for another three weeks after that, what are you going to do? Throw away any credits you earn the following three weeks, or just not play the game until the car is available again?
And this loop will continue with all the other cars.