The problem with theorizing about Fischer vs. Carlsen is:
Fischer was hugely insecure. If Fischer in peak form was to face Carlsen at his peak, Fischer would have found an endless string of excuses for avoiding playing him.
Strange how as time moves on all the negatives about the man are sloughed off until all that remains is positive and fanciful.
Fischer had a brilliant talent but is was embedded in a deeply flawed individual.
Why not 20. ... Qa5?