Technology evolves and in time hardware improves to a point where the performance difference between native code and something that runs through an intermediate layer is not noticeable in a user interface (although there will always be a difference, it just wont be be relevant any more).
The mobile web rendering performance might never hit the 0-grade latency standard where we put native, but it will eventually get so close to it as to not make any difference whatsoever to anyone but cold-hearted scientists the likes of whom whish they could keep their science drinks at absolute zero chilly bins.
If your argument is more in the line of why do games keep getting bigger and meaner, needing more resources, when we could have stopped advancing 3D a decade ago and every computer would be able to play any game today; then that's a whole other discussion. But I like the path towards true virtual reality with photorealistic graphics.