Sometimes, a person will play this video game. They'll select a character, choose a costume/color, and load up for a round of party brawling fun. As the match starts, the connection between the player and the character begins to grow. It's a fragile stage, where any action causes an extreme reaction in the eyes of the player. In most cases, the player will have some fun, in other cases, the player will be furious. In the very few lucky cases, the player will have established a wonderful bond with the character of their choice. (Note it doesn't have to be this way, average to horrid first encounters can grow as well!)
As the player continues to enjoy this character, their bond strengthens. Their power combined becomes more solid, and a true pairing begins to see the light. This light leading the connection to full blossom. Once they reach that stage, the need for other character choices is banished. Finding their true soul mate for the adventure that is; Playstation All-Stars Battle Royale.
-They are training for ranked
-They have multiple 999s and like to switch off every so often
-They only have one main and don't bother to get another secondary
-They enjoy that character a lot
-They are actually using another character and just don't want to get too rusty with their main so they go back to them for a few matches
Anyway, I don't really think that people should be forced to use a new character once they hit 999. You may think that using the same character all the time is boring, but somebody else may be having tons of fun doing that. I have Ratchet, Cole, and Jak at 999 and those are the only characters I really use for the most part, and I don't really get bored of them. If I really get bored of one, I'll use another for a while.