Most often it’s much easier to steal from the future, which is abstract, not yet instantiated, and at a disadvantage in defending itself, than it is from an extant party in the present that has more ability to fight back.
With the rent freeze and kicking the can on pensions, Mamdani is stealing from the future. Whether he recognizes this (given his bottomless entitlement) or not is unclear, but it does seem that one key feature of DSA politics is an ideological vision overdependent on immediate gratification and lacking the capacity to imagine the future in even the most basic way.