Ah, go out and buy yourselves some nice old motorcycles and some nice old two stroke lawn mowers... Better yet some 2 stroke Suzuki Water Buffalos! All of which will put out more emissions than your average 70s beater car with a functioning cat. That lawn mower will put out if used an hour a week all year long more bad stuff then driving a 90s Accord 12k.
I can't say I agree with the mileage limits for actually accomplishing cleaner air as you take your oldest cars on the road (which are usually owned by the elderly on very tight incomes to begin with). These cars are driven very infrequently but when they are, the drives tend to be short non warm up trips. So in reality these are the vehicles which need to be off the road, just don't tell Grandma she needs to fix her beater or get it off the road.
Cleaning up lawn mowers is cheap and easy compared to testing cars as you simply force the cost of making it onto the manufacturer. From what I've read you can cut lawn mower (and other small motored emissions) by 2/3rds for less than $30 a motor. Sure that cost would be passed onto the consumer BUT to me $30 every 5 years or so per item is a whole lot cheaper than $60 annually per car assuming you pass and in reality will clean your smoggy cities up more anyways!