So the first person to get a million coins put the video of what happens once you accomplish this. You would think that it would be a huge deal to accomplish something like this but...well...just watch the video...(skip to about 5:15 if you don't want to watch him collect the remaining coins).
So you get Mario and Luigi saying "Congrats!" and a new title screen. Is this what you would expect or want to happen after completing a huge achievement like this? Personally I think it is underwhelming but I haven't been impressed by any of the "New" Super Mario games for a while.