A quick google search found me a list of comparable luggage trackers (https://www.travelandleisure.com/best-luggage-trackers-and-smart-tags-6826825).
If you scroll down on that link, the 'LandAirSea 54 GPS Tracker' seems to fit the exact description I'm going for.
It's small/black, has a high power magnet built in (makes it easy to stick under cars), 4G LTE SIM card capabilities (allows for data to be transmitted once every 3 seconds), and has a battery that can last 1 week - 6 months in low power mode... All for $30 which is the same cost as an Apple Airtag.
Your point is that you shouldn't have to defend yourself because apple made an easily exploitable tag... Which is valid; but my point is very much true that stalkers who care about getting away with tracking you probably wouldn't use a tracker that alerts people It's there if you have a certain phone or get an app. They'll use one like I described or one more capable/expensive and track you with that. This technology isn't as new as you think it is 🤷‍♂️