Define "senior". Senior citizen developers? Or you meant that title should mean stature? If you're still coding after being at it for >30 years then yes, you're worth that to help level up entire teams. But, yes, given the prevailing definition of senior, sure, they're worth nowhere near that.
$20/hr was a contract rate. Your 1/2 million presumably is a salary (base). Not the same thing. $20/hr is roughly equivalent to $20k salary. $200k salary is reasonable for a competent developer who can hop on adding features to Gimp.
In my reality no developer (even a sh*y one) makes $20k.