The OTR price of my car is £34,995. After 4 years and at roughly 40000 miles, is it realistic to expect this car to sell or trade for £15,000?
I think it is more than reasonable to expect that, for several reasons.
In 4 years time we will be 4 years further down the road towards EVs. There will be many fewer negative anti-EV voices and sentiment as more people switch and find they actually rather like it. Demand for used ICE vehicles will be minimal and severe depreciation will hit all ICE vehicles, especially as more and more cities restrict where and when polluting vehicles may travel.
Demand for used EVs will be strong as more people wake up to wanting one, but supply of used EVs is limited to what has been sold right now. Whilst many EVs might be relegated into "2nd car" role by virtue of limited range, charging speed or both, the e-Niro will still be solidly >200 mile range and able to charge faster than many alternatives. That will mean that the residual values for the e-Niro should remain very strong relative to many other cars.
Currently this is evidenced by looking on autotrader and seeing that the very cheapest e-Niro 64kWh on there are 2 years old with over 20000 miles and asking prices are just shy of £30k from private sellers.
Looking at other 2017 EVs on autotrader, many are still at 40-50% of new list price at 4 years old (and more than that of whatever discounted price they were sold at), and that's for more range-limited vehicles than your e-Niro. Discounting of new cars can affect used car values, but the e-Niro hasn't really been discounted so that seems unlikely to apply.
2 years ago today I placed my order for a Model 3 SR+, which was delivered in august of 2019. It's now worth about £34-35k on autotrader prices, against the £38290 that I paid for it brand new. After I ordered the model 3 I sold my Leaf 24, which I had bought used in November of 2017 for £11k, for £10650. My experience so far has been that EVs depreciate significantly less than ICE cars, and the market forces that are causing that (demand for EVs rising, base prices of new EVs not falling) aren't changing at the moment. Even when some cheaper EV options come to market, like the Dacia Spring or Citroen Ami or the short-lived mii/citigo/e-UP refresh, then so what? People aren't cross-shopping dinky city cars against a genuine family hatchback.
The used car market isn't always rational, of course.... one of my colleagues once bought a pre-reg Seat Exeo ST with delivery mileage for less than a 3-year old Audi A4 would have cost him - which was literally the same car but for the headlamps, grille and badge (but the Seat had the latest engine spec and the upgraded interior from the A4 Cabrio). However, I think that the e-Niro will be a strong car on the used market for its range, warranty, reliability, etc.
As previous posters in this thread have alluded to in one way or another - we don't know
. However, I think we can make a reasonably educated guess.