I don't know if there was a pet system in GotW but even if there was it probably wouldn't be useful to use backuped files because GotW was completely different from LotS. In LotS there never was a real Pet System. The first thing that was released was the Monster Summoning System. I think the easiest and fastest way to implement a Pet System would be to change the current system from monster vanishes after x seconds to monster stays but all abilities will be disabled and it just follows you after x seconds. Like this people could use them as a pet and it's faster to develop than a completly different pet system (and will cause less confusion). But I guess the kRO dev team will develop a buggy new system nobody really likes as they always do
The point is developers of games often back up files. I know GOTW was different (in character design, gameplay, and even graphics engine, among general mechanics, and a few other things), but the fact is, there is the possibility of such files existing, even if they weren't implemented. Whether or not a plan is scrapped,, they do tend to keep the backup files. Reason being is to revert the changes, such as if the new changes happen to cause a massive bug, for instance, or if players dislike the new change over the old implementation. Some files never even get implemented, but are saved, so there is really no way of knowing if such a system is easily possible without just asking the Devs.
Yeah, a new system is likely to take more time, and bring more bugs. I still think that having two options for each monster you capture would be fine (and probably the easiest way to implement it, since it should only involve changing several strings of coding on how the monsters currently function). They could be summoned without any abilities, and not be able to attack, but would just be there for looks. You'd have to feed them every hour, though, which could bring profit, as they could sell the foods in the Kafra shop, 10 for 100 Kafra Cash, while making a quest to make two at a time for everyone who doesn't wanna pay (as long as the quest isn't too long). The monsters could "hide" for an hour and you'd be able to not summon them for an hour if you didn't feed them every hour. It would be an interesting system, in my opinion.