No graph can show action ('taken') so as to demonstrate causation. That's why so many arguments are susceptible to "correlation/causation" trouble. As you suggest, there are any number of causes for the data given. To seek proof of causation from a single graph is a flawed venture.
But if we look at whether market share was reduced ('away'), an observation may be made from a single graph. Not to say 'why' or 'how much', but the observation is still valid. If you sell 90 cars last year, and 110 cars this year -- and I sell 5 comparable cars last year and 50 comparable cars this year, I have "taken market share" from you in a generally accepted sense. Within the market space of two, I have more share of our market this year than the previous, even if your sales did not decrease. It doesn't matter how I did it (cause). It may not be true next year (correlation). It may not be universally true -- if there were 500 of this kind of car sold last year, and only 400 cars this year, your share of the total market increased.
My conclusion before, and again: "the mix has steadily changed. If that's not market share loss, what is it?"