>>280236795
The cynical answer is that it's just a matter of business. As long as there continues to be an audience that buys their anniversary/nostalgia products, there's no reason for Bandai to stop creating new ones. The singers are professionals, they'll return when there's a paycheck, and they're clearly not being held hostage by contracts (well, maybe the Lantis affiliated ones are) considering that they have all gone on to have their own independent careers by this point.