What we do know is that people are now unsure of their futures and not spending as much as a result.
That is just your inference from the figures, and deals with comparing just three years.
Even when there were unexpected increased sales in 2016 & 2017, no one would possibly have been justified in expecting the increase to continue indefinitely. Sales have fallen back to (above)their 2015 level. So what?
Without those two years we would be looking at the highest car sales figures for over a decade.