If you ain't advocating buying,it's great but i didn't find anything like this mentioned explicitly here, so I made a general assumption. Sorry for that!

Secondly, when you speak of keeping an animal as a pet, it is primarily for the owner's own selfish interest (like guarding, companionship etc. you had already mentioned here) whereas adoption of a rescued animal does not have an owner-pet relationship. It's more of an adopted-child & guardian / parent kinda relationship. But still, whenever you are domesticating an animal, inspite of all welfare measure you take, you still try to put several controls on them and make them behave as per your own wish. You are also responsible for separating them from their own family & tribe. They are generally provided an unnatural environment and diet. It's not about more cruelty or less cruelty but cruelty in intrinsic in domesticating any animal.

And I dunno why you feel so hard to argue about vegan thingy! All this food chain logic is a mere defense. Almost all of the pet "food" is prepared from farmed animals. All animal farming is a continuous torture and exploitation of that animal right from his / her birth until it becomes someone else's food (mostly in her very young age). There's no natural food chain involved. Victim has no opportunity to make any defense for her life. The slaughters involved in hatcheries for breeding those farmed animals is also not taken into consideration for this food chain argument.