I doubt most people desire being a criminal as an end in itself. Presumably car thieves either went onto the next best job, whether that be a crime (where "best" considers costs, like the risk of being punished, and benefits), or a legal job.

Or stayed at home and claimed unemployment, if the marginal gain from working ceased to be worth the cost.

Most types of crime have declined meaningfully in the last 25 years. The only segment I can think of that might have picked up some of those former car thieves, would be the drug trade (from pot to pharmaceuticals), as it did not see a decline.

With recent legalization of weed, I'd be interested to see what happens to those figures. At the very least I'd expect to see a lot less people getting arrested for having small amounts of weed on them.

(note: I watch Cops, main source of information there. Not sure if elderly bearded dudes wearing schoolgirl outfits for giving up fake names is representative of the US though).

They might specialize in other drugs like cocain, heroin, MDMA, etc. This kind of drugs may never be legalized and there will probably still be people buying it. But it might harder to enter in this market as people selling it are often quite organized and violents (apart maybe the last seller on the chain).

Some of it weren't in it for the profit. One of our cars was stolen some years ago, and found the next day a few miles away. I believe that it was some dirtbags who wanted to get home without hailing a taxi or walking.

maybe it's not much that people change jobs as much as new people that don't pick up the trade?

I.e. you never start stealing cars cause it doesn't seem as lucrative and is riskier than just selling drugs.

Sending spam and scam mails?