Instead of legislating that all new cars have to have a catalytic converter fitted, why not simply state the acceptable level of emissions and leave it up to the manufacturer how they achieve it.
Several modern engines (Toyota Carina E and Rover 'K' series are a couple of examples) were designed as "lean burn" engines, and could have easily passed the emissions tests of the time with no cat fitted.
However, the government insisted that these engines all be fitted with a catalytic converter.
This meant the engines had to be de-tuned to work with a cat, killing the mpg and performance, and installing an expensive and unreliable component into a car which did not need it.
If you want to control emissions, set limits and enforce them.
Don't assume that blanket legislation is the correct answer.
Why is this idea important?
Catalytic converters are the world's biggest user of the world's platinum. That platinum isn't always recovered – a considerable proportion of it is lost through the exhaust.
Cats are not green – see above.
That "rotten egg" smell they produce is hydrogen sulphide, which causes acid erosion of buildings. It's not nice stuff.
Insisting of cats suppresses design and innovation – there are different (and better) ways to control emissions from vehicles. Let the engineers find them!
A catalytic converter is an expensive item. The cost of replacing one could easily push a perfectly serviceable car into an early trip to the scrapyard. How is that "green"?
Many people remove them anyway – just look at the black market in de-cat pipes.