For one, there are some important differences between AirTags and a GPS tracker. AirTags don't use GPS (at least, on the tag itself). They're tracked by all the Apple devices around them, and they can use lower power (perhaps even passive) methods. A GPS tracker requires some kind of uplink. A tracker using GPS would need to be larger, and have a shorter battery life for a given size. AirTags are also innocuous to buy and not immediately alarming to see in someone's possession.
The argument that anyone is capable of building a GPS tracker in their living room seems unlikely to me. I think this is far beyond the capabilities of 99.9% of people (although it's a fairly straightforward and very fun project for people interested in Arduino).
All this said, I am mostly in agreement that the arguments against AirTags seem fairly weak. Especially because I assume Apple has a way to easily determine who placed the AirTag, and that they would face serious consequences.
All that making GPS tracking devices illegal would do is to eliminate all the positive-sum uses of the technology, while creating literally no additional barrier for people already willing to break the law.
It’s the “then only criminals will have guns” thing, but much more clear-cut, because while making your own gun from general-purpose input components requires a machine shop and an unethical person with machining skills to run it (and so making guns illegal really does make guns harder to acquire — though not impossible), making a GPS tracker from general-purpose input components can be done in your own living room without even a soldering iron.