If Apple is working on a car it is very unlikely to impact Tesla's growth opportunity for a very long time (15 years or more from now if ever). Tesla has multiple moats around their growth opportunity.
Consider this moat:
There are two scenarios in Apple developing a vehicle,
1) Take advantage of Tesla's offering of their intellectual property
2) Engineer their own solutions without use of Tesla's technology
It's worth remembering Tesla did not relinquish their patents... they just said anyone is free to use them in good faith. I do not think Tesla would consider another vehicle maker creating long range EVs with a separate walled off SuperCharger-like network as being in good faith. Thus, I think if Apple makes a move to EVs using Tesla's patented technology, scenario 1, Apple would join in on the SuperCharger network. It would take relatively small amounts of money from Apple's perspective to make the network far more dense, which would make Tesla's vehicles more attractive to consumers.
If Apple were to choose not to use Tesla's patents, my gut estimate of a best case scenario of them developing as good a technology as Tesla's on their own without stepping over any of Tesla's patents would be about 2020. So, perhaps they'd be ready to bring their first vehicles to market in about 2023 if they go with scenario 2. Maybe at the start they'd have a few hundred thousand vehicles produced in 2023, and try to grow from there.
Here's another moat:
Staggering amounts of capital to transition to EVs... even for Apple.
As an extreme example I give less than a 0.1% chance of happening, if Apple were to use all of it's current cash on hand (~$180 billion) to begin building auto factories and Gigafactories starting in ~2019 (assuming they get an initial car on the market in about 2018 to prove to themselves they can go with this "all in" strategy) all $180 billion or so would be roughly enough cash to build auto and Gigafactories suitable to supply 10% of the 100 million vehicle market when those factories might be at full production capacity about 4 years later (assuming each GF cost about $5 billion, each Fremont-like 500K annual vehicle capacity auto plant about $4 billion).
If Apple had 10% of the global vehicle market in 2023, Tesla would still have 90% of the market to sell to. Moreover, that was an extreme that simply wont happen, as
-Apple is a corporation owned by stock holders. I think it is enormously improbable their management, which ultimately has to answer to shareholders, would make such a massive bet as to invest $180 billion (all of their cash) in an industry they would at that point have one year of experience and 0.2% market share in.
-This would mean building about 20 Gigafactories and 20 auto plants in the span of 3-4 years. I find it extremely improbable such a massive undertaking could be pulled off by anyone, let alone a company whose never built an auto plant or a battery factory.
So, even in an extremely aggressive scenario which is tremendously improbable to be embarked on (and very improbable not to be a mess if somehow given the green light) Apple would have 10% market share, and Tesla 90% of the market to try to grow into (all this assuming Apple's car was more attractive than Tesla's to all consumers, thus Apple taking 10% share before anyone considers a Tesla).
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I do think it's worth considering that a slower more realistic size move into autos by Apple could eventually effect Tesla's growth trajectory, perhaps as soon as 15 years from now. If Apple were to get up to a few million vehicles per year by 2025 (and be making strong margins on their new business), this would push the other automakers to transition to EVs sooner, and might draw in other new players with rarified levels of cash on hand like Samsung, Google, or even some oil companies. So, perhaps, this could lead to something like 30-60% of new vehicles being EVs around 2030. Elon would love it, but with that kind of acceleration of the transition to EVs might lower the ceiling of how much market share Tesla could grow to in the 2030s and 2040s.
One last point- Apple could try to short cut the process of building infrastructure by buying existing auto plants, or another automaker or two, or three... That could be more efficient than building plants from scratch, but it would bring to Apple some of the problem the incumbent automakers have... creating losses in the outgoing ICE business, and existing investments in ICE infrastructure, engineering talent, and a workforce trained in ICE. I think the biggest realistic kind of move Apple might make could get them to ~3 million vehicles per year around 2025, or 3% of the market.