Reflecting the enormous effectiveness of free markets in the software space. If there was a strong regulatory system around software development it might cost orders of magnitude more to run a modern OS.
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.