Are you saying that your nozzle check print shows no defects, and that's why you are questioning the printhead status? I'm on my third printhead failure in four years of moderate printing with my 5000, but it's the MBK nozzles on the check, and I don't print any matte papers at the present. When the right fails, I'll probably go for a 6300. My first two failed with a month of each other.
So after five years, I'd say you have a good chance of the other head failing also - the original PF-01 heads don't hold up as well as the PF-03 replacement heads, and you've gotten good service from them. So you are looking at $900 for new heads and will still have a quite old printer. As all but your four grayscale carts are transferrable to a 5100, which comes with about $450 of ink left after priming (about 70ml of the 90ml starter carts) it would make good sense to me to go for the 5100 now rather than curse your luck when the other fails and you're already $450 in the hole. Although not quite up to the new x300 inks, the x100 grayscale inks do decrease bronzing and gloss differential while increasing surface toughness. As for a 5300 in the future, who knows? Personally, I would stay with Canon with it's more frugal ink usage and freedom from annoying clogs/cleanings. The 5100 is said to use less than the 5000 for routine head maintenance.
Good luck! Pete