I realize the 5 can never have "pulse o2" because it is missing the hardware but I don't see why it can't get basic features found in the watches costing half as much?
IIRC DC Rainmaker suggested the F5 would be getting body battery, as it's simply a calculation using the metrics already collected.
However, give that it's a FirstBeat algorithm (as is the new VO2 max calc and Respiration Rate calc - also possible given the current hardware), it might be that there was an additional cost for licensing this and Garmin have decided not to pay it. This also creates a clearer (partially artificial) difference between "old" and new products. There isn't much new on the hardware side of things in nearly three years.
It looks like even the F5+ won't be getting this stuff and that is a 9 month old product!
given how huge and international garmin is, surprised they don't just buy firstbeat outright and save all the licensing costs for all time and deny the features to their competitors