Bangkok has many problems with taxis. Part of it is that the fares are set unreasonably low, so drivers try to drive >100h/wk and are very poorly trained. Market pricing would probably be higher for at least some taxis. (it was bad enough that whenever I didn't want to be seriously hassled, I'd use a hotel car, at a negotiated rate. I'm fine with spending $20/hr for an S320 with driver and then tipping him 100%.)
The big problem with regulation in general is that it sets a single value, static in time and across preferences, as both floor and ceiling. For some things, that might make sense ("levels of acceptable poison in baby formula"), but for other things it doesn't make sense ("housing").
For Bangkok, I think the right thing would be to encourage public transit over taxis, but to encourage taxis over private cars. Charging a per-day fee to have a vehicle inside the city (similar to London), combined with actual traffic law enforcement, open access to getting a taxi medallion once you pass minimum qualifications, and either market rates or at least fair rates for taxi rides, would probably go a long way.
If taxis substitute for several cars, increasing taxis should decrease congestion. That is the London model, but London taxi drivers are kind of the polar opposite of Bangkok taxi drivers. Other SE Asian taxi drivers are fine, though -- Hong Kong and Singapore -- and it's easy to convert taxi fleets to NGV or Electric compared to private vehicles.
While I'm pro drug legalization, testing for meth in drivers would probably be worthwhile too. I always checked the eyes of any driver I used, since so many of them were tweaked out of their minds and thus even more unsafe than usual. (I actually had one driver assault me, too, which was kind of funny afterward).