But cars are a good example, say your automatic shifter breaks. There are no real third party options, you've pretty much got to buy a new one from the manufacturer or by used one. You can still do all normal car things with it you just can't use third party pieces. Same with the lights, they still work, they shine light and do everything that they did before now you JUST can't use third party bulbs with the system.
If you want to go with other shitty analogies say you made a game, something like an MMO that has monthly fees. You sank a lot of money developing the game, the box price isn't high because you know you'll make your money from the monthly fees. You also use money from the monthly fees to improve the game, fix bugs, add content. Now your subscription model is a little strange, it works by players entering a code on your website. They buy it from your online store, get it in their email and use it to get play time. Now let's imagine that your security isn't absolute and some third party figures out how your keycode generator works and they start selling codes for half the price. Now your paying subscriber count is shrinking. What do you do? Do you A) fix the problem which results into many users losing money since they already bought several months worth of play time from this company? or B) leave the system as it is because openness or something like that?