Every engine would have the same max power, but the better manufacturer could still have more torque at lower RPM, lower consumption (and so less initial race fuel weight) and so on.
Moreover if the max power increases year by year engine manufacturers could improve till when the max fuel flow become again the limiting factors, but until then some other engine manufacture could join the F1.
After al when there was the NA engines they FIA introduced a maximum RPM limiter firstly set to 20000RPM, then to 19000RPM and then to 18000RPM and for a NA engine with fixed displacement and same architecture when you limit RPM is as if you limit max power. So what was the point of being an engine manufacturer in the latest NA years?