You do realize that "tied to" implies that you can't change jobs? But that's not the case with H-1B workers. You can switch jobs at will, and even work multiple jobs (with concurrent petitions) at the same time.
Furthermore, AC21 enables you to begin working as soon as your next/future employer has filed a petition – so you don't have to wait until USCIS makes a decision. With a good employer and attorney, USCIS will generally approve the case (although I'm sure there are outliers).
In practical terms, the effect of AC21 is that (for the most part) it allows you to take up any job in that is directly related to what you major. The employer has to spend a bit on the petition and on an immigration lawyer, but in our industry it's a drop in the bucket compared to what they'll be paying you annually.
I'm 99% certain actually being on the payroll of multiple companies while on H1B is a felony. You can still be working for your original employer while your petition with the new one is underway, or start working with the new one once you receive your petition receipt, but definitely not both at the same time.
I'm not sure what industry you're in, but 20k out of a software dev's 60k-100k salary is a very very big drop, especially for a junior/fresh grad hire. And of course, 20k lump sum is out of the question for many startups.
Furthermore, AC21 enables you to begin working as soon as your next/future employer has filed a petition – so you don't have to wait until USCIS makes a decision. With a good employer and attorney, USCIS will generally approve the case (although I'm sure there are outliers).
In practical terms, the effect of AC21 is that (for the most part) it allows you to take up any job in that is directly related to what you major. The employer has to spend a bit on the petition and on an immigration lawyer, but in our industry it's a drop in the bucket compared to what they'll be paying you annually.