But isn't the variation in design also interesting in and of itself? We already have a dominant car but everything is the same.
Also, allowing more freedom in other areas also allows for fluctuations in competitive order depending on other factors- e.g mclaren's Honda v6 engine with all kinds of hybrid tech is not as good Ferrari's v10 with just kers but they get a set of Bridgestone quali tyres that work brilliantly at Barcelona and much better than Ferrari's Pirellis; Mercedes' v8 is somewhere in the middle but because they've managed to pass the crash test whilst maintaining a high nose (maybe the nose deforms enough to prevent the whole car from lifting up in collision) their rear downforce advantage makes up the difference.
What I'm saying is the regulations would all have to work in this way, not just re power units. And yes, one car might still get everything right but I still think it would allow other teams more avenues to catch up, with customer chassis helping keep costs down.
And worst case scenario, one team still wins all the races but we get to discuss lots of diverging philosophies