While I don't doubt that regulatory bodies and/or ethics boards would have some cost effect, I don't think it'd be to the degree that you suppose. If anything, the long-term costs might be less. For example, say you take bad legal advice from someone online, which you follow, and now you've been found liable for some misconduct and have to pay a fine. Whereas, if you had paid for an actual solicitor and followed their advice, the initial cost would've been higher, but you wouldn't have had to pay the higher cost of the fine.

Similarly, it might have preempted things like the $99 Apple Developer Program: if your software-engineering licence is at jeopardy, you may be less likely to upload malware, or target micro-transactions to children, or make cars lie about their emissions, etc. How many lives have those ruined?

It might help with the AI-slop crisis too: Speedy Web Compiler might not have needed to adopt a policy of rejecting every external PR out of hand if they could have chosen a "licensed software-engineers only" policy instead.

Some other things to consider too like what effect this would have on anonymous/pseudonymous contributions, and whether the regulatory body/ethics board would accommodate that in some way. There's been some discussion about how governments flirting with age verification laws could do so without forcing everyone to dox themselves to every website they use, or allowing the government to track everything they do. Perhaps a similar system could be adopted for contributions. Etc.

Overall, I do agree that the upfront cost would likely be somewhat higher, but I think the overall cost to run a modern OS would be somewhat lower, resulting in a similar-enough cost overall. Though this is just my opinion based on vibes. I'd love to see any data about this.