A 4G modem would probably be overkill, but one might be able to do it for free with lorawan. Or alternatively a big label saying "this bike is GPS tracked" with absolutely no extra hardware at all and hope that bike thieves are dumb fucks.
NY:
https://nyc.streetsblog.org/2024/09/12/the-river-wild-bikes-...
Michigan:
https://www.mlive.com/environment/2024/08/seriously-waterway...
DC:
https://irrigationleadermagazine.com/not-so-convenient-why-s...
Ohio:
https://www.nbc4i.com/news/local-news/columbus/30-electronic...
My bike, on the other hand? A thief could just find and remove the tracker - or strip the bike for parts.
But yes I do partially agree that it probably wouldn't help much, since it's more about the thieves not wanting to mess with a company with funding and a legal department behind it vs. some random bloke who they can rob with impunity. Private bike security firms might be an answer to the issue, you pay a monthly subscription, they put their own trackers on your bike, employ licensed people who retrieve stolen ones at gunpoint and guarantee protection up to some total cost. Probably not cost effective though I guess, given that the police themselves have given up.