Jailbreaking uses vulnerabilities, there is no way to jailbreak a phone officially. Whereas Android allows sideloading apps officially.

There is also no official way to convert your car to diesel/electric...

Of course there is. At least in my country, there is a formal legal procedure to get your car inspected and re-registered after any changes are made.

It voids warranty though, doesn't it?

But there's a procedure enabling you to do it. On iOS, there are procedures preventing you from doing it.

The car doesn't have cryptographic locks on its bonnet.

Unless it's a Tesla.

At least with a Tesla there's a healthy aftermarket using the motors and batteries for vehicle retrofits.

But neither does the car manufacturer actively try to prevent you from doing so.

Jailbreaking voids my warranty, could be impossible, and disables a lot of features (OS security updates). It is unreasonable to compare this with another app store.

Modding your car from petrol to Diesel also voids your warranty.

It's not like car manufacturers go out of their way to update their new models with anti-conversion technology with each new release.

Unless the car is a piece of agricultural equipment, in which case, we can see exactly why walled ecosystems are bad.

That's a good point. Many farmers are as frustrated and stymied as many iphone users. Locked out of repairing a device that their livelihood depends apon.