I don’t think there would be a huge pull to older cars because they have always been significantly cheaper to run in terms of total cost. People don’t buy new cars because they are cheaper to run than an older car. They buy new cars because they are new and shiny.
This is skewed somewhat by the fact that if you're looking at an EV as your only car then a newer one is likely to be the only option - there's a 10 year old ICE out there to suit almost everyone's needs. A 10 year old EV is going to have very limited range and slow charging (unless it's a Model S), which means for most people it's not going to be an option.
However, if you're saving £200+/month in fuel, then that effectively increases your purchase budget by £200, so if you'd only previously been "spending"* £100/month on the car itself, you could now "spend" £300/month for the same total cost.
When I got my Zoe, it was a choice between running an old banger worth ~£3-4k until it "died" (so about 3-4 years), or a newer EV. Given I was spending £200/month on fuel at the time, plus £15 VED, along with the depreciation "cost", that was a total of ~£300/month, not taking into account any inevitable repair costs for a 10+ year old car.
HP on the Zoe was £1k deposit + £360/month, plus ~£15 in electric - but after 5 years it would still have been worth a few £k (looking on Autotrader, around £8-10k at the moment).
So for 5 years:
TCO of the old banger = £3k (purchase) + 60 x £215 (fuel+VED) = £15,900
TCO of the Zoe = £1k (deposit) + 60 x £375 (HP+electric) = £23,500 - £8k (equity) = £15,500
So almost identical overall cost over the 5 years.
Yes, I could have spent half that on say a 30kwh Leaf, but the 80-100 mile range would have been a right pain in the **** (to put it mildly!).
Yes, I could also have gotten HP on a cheaper ICE, but the running cost savings wouldn't have been anywhere close.
Those maths don't necessarily work any more since the government grant is gone and it relies on finding a 0% HP deal, but with "newer" cars like the Niro and MGs dropping into the £15-20k bracket, it's still feasible to get a much newer EV compared to an older ICE, without increasing your budget too much. Start hitting those newer EVs with big VED bills and people will stick to the older ICE (and obviously not everyone is in the position to get £20k+ of finance at any kind of competitive rate!)
* e.g. purchase price vs depreciation/lease cost/etc.