In my experience most systems are a series of tradeoffs. Even though C4 has evolved over 70 times, are there advantages conveyed by C3 or is C3 just easier to evolve?
C3 is the ancestral system - 45 million years ago, all plants were C3. C4 is a relatively new evolutionary innovation.
The evolutionary history goes something like this. 3.6 billion years ago, Earth's atmosphere was very rich in CO2 and had basically no O2. Then photosynthesis evolved in bacteria in the ocean, using the enzyme RuBisCO to trap CO2 into sugars. Those bacteria became incredibly successful and in the process released a lot of O2 into the atmosphere - over the next 2 billion years the atmosphere became oxygenated. Unfortunately, RuBisCO sometimes mistakes O2 for CO2 and accidentally creates a toxic product that wastes energy instead of storing it. So this new oxygen-rich atmosphere started to make photosynthesis more efficient. Eventually (42 million years ago) the atmosphere became so oxygen rich and CO2-depleted that this inefficiency was a major evolutionary pressure, and C4 started to pop up.
There's very little compromise with C4 - it's just better in a very wide range of commonly occurring environmental conditions.
In my experience most systems are a series of tradeoffs. Even though C4 has evolved over 70 times, are there advantages conveyed by C3 or is C3 just easier to evolve?