It's very unlikely he will resign. Just as it wouldn't do the U.S. much harm if congress took a vacation for a few years and forgot to pass any new laws, it wouldn't do the Church any harm if the Pope were to fall ill for a few years and didn't write any encyclicals. This Pope has written more great encyclicals than any ten other popes. I quiet period in the Church wouldn't do any harm, and if God doesn't want that, He can take the Pope to himself any time He decides.
Dante called the action of a medieval pope who resigned and was replaced by a wicked successor, "il gran refuto," and he gave him a place in hell in the Divine Comedy for resigning. No, I think this pope will patiently accept whatever God sends him.
If Parkinson's takes its normal course, JPII will become incapacitated at some point.
Who will be "running" the Church?