The value of X is determined only as the triggered ability of Rubblebelt Rioters resolves. Once that happens, the value of X won’t change later in the turn even if the greatest power among creatures you control changes.

If Rubblebelt Rioters is the only creature you control as its triggered ability resolves, X is equal to its own power. Unless another effect or a +1/+1 counter has raised its power above 0, this won’t have any effect.