I reckon that because demand is growing therefore X being sold now but in 3 years time the demand will be 2X. So twice as many people looking for a 2nd hand one as have been sold right now, so supply-&-demand will keep 2nd hand price strong. I think this has been the case all along (plus things like £Pound weakening and supply problems both driving up price of new, and dragging 2nd hand with it). I sold my 2015 MS 3.5 years and 95K miles later for 50%. In 2015 an MS purchase was nudging £100K ... any equivalent-spend BMW etc. ICE would have been "worthless" after driving it out of the showroom.
One thing that would rock 2nd hand price is if a brand has a "snag", given the newness of the Tech. Tesla buying batteries from a variety of sources might have that problem (as GM Bolt has done), but it is more likely to hit others who are new to all the EV stuff - albeit having had the benefit of taking a Tesla to bits to see how it was done before they started.
