Index futures are among the most liquid and highly traded instruments and have no natural expiries, a perpetual might make sense. There is a direct corresponden

Index futures are among the most liquid and highly traded instruments and have no natural expiries, a perpetual might make sense. There is a direct correspondence between the rates component of the index futures and all other naturally expiring instruments (options, bonds, commodity futures) so there’s an angle of “yet another weird rates product everyone has to care about” if you add perps.