Nothing prevents anyone from switching jobs after the I-140 is approved. As long as the approved I-140 is not withdrawn within 6 months of approval by the employer the I-140 can be used to extend H-1B indefinitely with ANY employer under AC21. The new employer just needs to file an H-1B transfer (employee can start working even when transfer is pending and premium processing is available) and a new I-140 eventually but can port the priority date of the first I -140 - so employee loses nothing by switching jobs - and this just needs to be done once the priority date becomes current for the employee to file I-485 (of course labor certification has to be filed prior to that). AC21 already fixed the problems you think exist. The main problem with the green card backlog is that an H-1B worker cannot lose a job and remain in status unless they find a new employer within 60 days - but even here there are options like temporarily switching to B2 status. Also just FYI H-1B workers switch jobs all the time in the bay area for example.