tesla has been adding about the VW mkt cap PER DAY for a few weeks.
to justify based on TAM, the global mean price of cars has to double, and tesla needs to capture that at ~50%.

That may be true, and if Tesla's future prospects were limited to selling cars for about $50,000 each, there's no way you could get to Tesla's current valuation. VW has almost 10% of the world market, so a couple of weeks of increasing by one VW mkt cap per day gets you over 100%.

But Tesla has much wider ambitions. They hope to corner the market for self-driving, build robotaxis, generate and store electricity, and other things. Musk said that the personal robot they are developing has a larger TAM than cars, and a better profit margin. Whether it all works out anywhere near these ambitions is highly debatable, but Tesla shareholders (I am unfortunately not one of them) are not basing their analysis, such as it is, on the TAM of cars.