There's still a massive financial difference between a £30k new car (however you pay for it) and a £10k or cheaper older car
I have an 18 year old Ford Focus which I use for certain leisure journeys, doing 2000 to 3000 miles a year as for some trips, public transport really doesn't work well, and that's for someone living in London which has far better public transport than the rest of the country. The family car is still massively convenient even if you don't use it that often, especially if you have children, elderly parents, need to take stuff to the tip, want to go to the countryside, self catering holidays etc
Indeed, a £30,000 car is likely to cost something like £5,000 a year, whilst a £10,000 one would be cheaper, but still a fair amount of money.
Let's assume that you but a car for £10,000 and then sell it for £2,000 8 years later that's a cost of £1,000 per year in just purchase costs. Now compare that to a season ticket that's likely to be about (given that you're doing an average of 12 miles a day if the car is for work purposes) £1,000 a year then you've got all your other car ownership costs you still have to pay.
Let's assume that's £1,000 (fuel costs, insurance, MOT, maintenance, VED, etc.), that money could be used for other travel options over the year. For instance hiring a car at £35/day that's 28 days of car hire that you could still have access to a car, therefore with fuel it'll be a few days less than this. At 20 days use then that's an average of 150 miles a day to hit 3,000 miles a year. For sorter trips (say 7 miles) a taxi would cost a similar amount.
However, £1,000 for all your other car costs isn't a lot, not when your likely to be spending £250 on fuel (if not more), tyres need replacing (even 2 full sets over 8 years at £200 each set is another £50 a year), VED, servicing, MOT & maintenance cost, insurance and maybe breakdown cover.
Even little things, like infrequent parking costs, can soon add up. For instance parking charges of £4 a month is the best part of £50 a year.
There's also the fact that, whilst in lockdown cars are still costing most of their costs but being used hardly at all, unlike public transport where you only pay for what you use.
Now yes, is unlikely that knowing the true cost of your car is unlikely to result in people not having any car in their household, it is very likely to reduce the number of 2+ car households.
Especially where that's in conjunction with both halves of a couple working from home 2 days a week and then only needing to use public transport 1 day a week on average and some occasional other times when one is needed in the office on a working from home day.
Even a 10% fall in households with 2 or more cars would result in a ~2% drop in car ownership. Which may not sound a lot, but given that there's 32 million cars, that's 600,000 fewer cars on the roads.