I haven't read the book, but I have heard people say that the global wealth tax is presented by Piketty as an admittedly idealistic suggestion rather than an imminent policy recommendation. From what it sounds like, the global wealth tax would merely tackle inequality directly, even if it meant lowering the quality of life (or slowing the growth in quality of life) among workers.
Aside from him, no, I've literally seen not a single person defend the policy suggestion, even among reviewers who are otherwise completely positive on Capital in the 21st Century.