[dekhn]

It's not feasible to edit a majority of most adult cell types; it's not practical at this time. You'd need a very effective vector (high sensitivity and specificity). More likely, you'd want a vector that targeted neuronal stem cells and transform a large fraction of them. Those cells would then go on to make new, modified cells.

I do want to point out that were are absolutely very far from modifying germlines to eliminate most diseases. That's a common conceit, and as somebody who's worked in the field for 30 years: temper your expectations. A single person died in a single gene therapy trial 25 years ago and that pretty much stopped gene therapy in its tracks for a long time. Societal expectations for safety when modifying genes is far more stringent than any other medical technology.