Its more like the other way. Imagine a behemoth having to toss out all that IP, know how, patents, manufacturing, factories, assets, product lines and what not all to compete with a start up.
Why do you think Microsoft can't compete with Android? Doing so requires tearing the Windows Operating system and there by the whole company.
Imagine somebody like Volkswagen being made to do that.
No matter what happens, anyone wanting to build serious EVs will at some point be licensing patents, especially the stuff they do which really is cutting edge around their battery packs.
History is full of examples of this to be the case. Has Apple ever had a head start on any industry ?
No, and that is also why the iPhone is so expensive.
Almost everything in the iPhone has Apple paying some licensing fees to Qualcomm.
Late entry is not free.
Also, define Porsche:
"Porsche AG is headquartered in Stuttgart, and is owned by Volkswagen AG, which is itself majority-owned by Porsche Automobil Holding SE." - https://en.wikipedia.org/wiki/Porsche
I'm a bit skeptical whether Tesla will survive a behemoth like Volkswagen shifting gear towards main focus on electric vehicles.