The claim was that you could "
earn fm like that until game support ends". While level ups indeed offer another source of FM, the number of matches you need to win to level up increases with your level, but the reward does not. So you cannot keep earning fight money at a reasonable rate by playing the characters you actually like to play, you'll have to go out of your to play characters you are probably not interested in, if you want to keep the FM flowing based on level-ups.
But let's be generous and say that you can keep leveling up, on average, every 10 matches, even after you've cashed out on the quick sources of XP. That would earn you an additional 100 FM per match, on average. Let's further assume that you win 50% of your matches, so that you would earn, on average, 125 FM per ranked / casual match (rather than per win):
- Character = 800 matches
- Story mode costume = 320 matches
- Color = 16 to 80 matches
- Stage = 320 to 560 matches
- Title = 40 matches
- Profile design = 40 matches
You don't see a problem with this?