It’s not a successor to Hubble. Webb is an infrared scope and Hubble is a visible scope, and their are no plans to put a new visible scope up.
Elon Musk should offer to buy Hubble for a dollar and go save it. He could easily make his money back by renting time on it if he can figure out how to keep it up there.
Nah, the BFR has plenty of cargo space. Build a new, updated and much bigger Hubble that just fits on the BFR and launch that sucker.