Tesla has a (IMO rediculous) high market cap and stock value. A severe hack could affect that, which, combined with a short position, would allow attackers to make a large amount of money.
What I'm saying is: Tesla already is a valuable hacking target.
https://news.ycombinator.com/item?id=38932228
According to one of the comments: "The price of BTC initially jumped 3% on the hacked tweet. That's $25bn+ of market cap."
It's lifting secret keys that allow criminals to open any Tesla 'till an OTA update, or callback has been rolled out. For days exposing your Tesla+luggage to theft. Or just breaking stuff: imagine the flack of every tesla driver suddenly losing access to navigation or entertainment for weeks. Or camera access: imagine the outrage if hackers can record and publish anything done and said inside and around Teslas in the last months?
From a purely pragmatic perspective, Tesla's that occasionally and irregularly crash are a way better alternative than terrible human drivers.
The good(?) news is that e.g. WannaCry took out large, "boring" companies, like Maersk, or MSD. These companies typically have a low Alpha, low volatility, and people invest in them for long term: decades rather than weeks. Whereas with e.g. Tesla, every fart that Elon makes has an effect on the stocks.
While I don't disagree that Tesla is a valuable attack target, the reality is that for market value security does not matter.
There can be a massive attack on Tesla announced tomorrow, the stock will go down 5% for a few days and recover within a month. Nobody cares. It's a depressing reality.