I wonder how many pilots who left or were otherwise not working as pilots will be coming back?
Not many. A few who are well established in other employment have asked to have their training deferred - something the airline are happy with as they cant train everyone at once and there's far more wanting to start now than defer. One I know of, bought an excavator and established a successful business for the last 18 months and loved being home with family every night, but that doesn't pay the bills quite as comfortably and thus he took the first opportunity to re-train and get back to flying.
A few who retired or resigned are still in the industry flying corporate aircraft and other GA flying.
How many have found driving trains or trucks a much better option?
None that I know of and I know of at least a dozen. Some are doing a little casual driving as their flying rosters are fairly 'relaxed'.