Massive cost - to whom? Any testing won't be free to the individual Imo but around £85 per senior citizen over 70, and every 3 years for anyone agreeing to your OP, ceedee. If there was a massive cost I would rather see more effort directed at catching people using mobile phones.

For many people 70 is not that old to prevent them being perfectly able to drive well with good mental and physical ability, plus a lot of driving experience behind them.

Perhaps at 80 a test might be a good idea. My vehicle was smashed badly in the side last year by a guy who looked over 90 and was very.frail. He should not have been on the road imo and I expect the old booger knew it cos he drove off before giving me his details. So yes, maybe a retest late in life, say 80 plus would keep some of those unfit off the road.