Companies can grow in a shrinking market by stealing share. I’m no Tesla bull, but you’re casting a loose prediction with far more certainty than it merits.
It is a concern though, and as a Tesla supporter I’m a little worried. Tesla is also going to be looking at serious and sustained competition from giants like VW, so if they want to steal market share they’d better do it fast. I accept the argument that such competition is a good outcome for the planet, but I’m not so sure it’s a good outcome for Tesla the company.