Not hard for a non-technical person to remove the speaker. Housing pops right open with a small flathead screwdriver. Then you just remove the speaker's magnet, no desoldering or anything like that required. Took 5 minutes. I did it to one of mine to use it to track my bicycle if it ever gets stolen.
If the thief has an iPhone, they'll still be alerted that the tracker is following them. If they move their phone around on your bike, they'll be able to find it, within several inches, with the NFC identification feature that gives the last four digits of the owners phone number. Maybe Apple's campaign is working: https://www.theguardian.com/technology/2020/feb/26/apple-doe...
But against a professional thief an AirTag probably won't do much.