Devil's advocate makes me think the worst things that can impact current generation BEV values would be....
1. New generation batteries cause a step change in pricing/range and existing tech suddenly looks a bit lame.
2. A significant number of the cars suffer fires or troubles that really impact confidence.
3. Charging infrastructure does not get itself sorted (this one seems far less real but remains a risk)
I think the third issue is the most immediate risk.
All the battery makers are working on the first, Tesla has already announced a new battery that are makes them cheaper to produce and, Tesla says, enables them to offer five times the energy, 16% more range and six times the power of the current battery pack. They will be in cars during 2022 and will make the £20000 "Model 2" possible.
Car fires? no more of a risk than ICE car fires.
The biggest threat to BEVs is hydrogen fuel cells, we could be seeing an automotive equivalent of Beta vs VHS video wars of the 1980s. I have a Tesla, but I'm not assuming that BEVs will win over hydrogen fuel cells.