I think they may mess with companies - which is another reason I think it is quite possible he starts his own - but I don't think the WWE nor Bryan's health will stop people from booking him. The wwe will try to tie some companies up in the legal process just by having more resources, but if he shaves and goes by Bryan Danielson their ground is fairly limited on what they can do to stop people.
I do suspect that they might cut Evolve and Progress off for him though through their relationship with those companies, which would be a real shame.
To be clear, I am not saying I WANT this to happen. Ok, in a way I am selfishly wanting it to happen, but if i had any actual say in this I would 100% defer to his health and happiness. If he never wrestles again I won't complain a bit. I will be perfectly happy. When I think about what I want as a fan it is sort of in a vaccuum, without really thinking of results. That isn't because I don't care about his health (if there is a celebrity that I don't know at personally, but genuinely care about as a person, it is him), it is that I don't have nearly enough information and I don't think it is honestly any of my business what a grown man does with his body and his career. So, when I say "as a fan I would love this" it comes with the understood caveat of "if wrestling is what he wants to do and chooses to do with the information he has". Ultimately the "wrestling harming themselves" vs "enjoyment of the product" conversation is I think far bigger and more complex than this case; this is just the easiest case to filter it through.
I am just saying I believe he will wrestle again and I believe he will do outside the WWE if not inside and I don't believe that health concerns will stop wrestling promoters from booking him and watching money roll in. What I think will happen, what I as a fan would love to see, and what I think SHOULD happen given his health are three totally different conversations (three conversations that are getting interwoven here quite a bit).