Typical platy lifespan?

I've had a pair of female Micky-Mouse platys in my planted community 55g for just over 2yrs now. Over time, one grew to your average 2" or so platy, where the other one was larger. In fact the largest platy I've ever seen or kept. Super thick, wide and tall. Nice fish, great coloration and I never had any issues with them. Out of the blue, I noticed the smaller of the two hanging at the bottom and breathing heavily. The next morning I found her dead.

I did add 4 female bettas to the mix about 6 weeks ago, with no QT. Petsmart purchase. Running with scissors I know.:uhoh:
Tank has never seen even 20ppm nitrates between WWCs.
Fish get fed a variety of flake and freeze dried food.
pH always 7.6. GH and KH in the 107-125 range.

I know out of the blue deaths happen, but I'm just curious to hear others experiences on how long they've had platys live.

I'm figuring these platys were close to a year old when I bought them, so they may be a bit over 3yrs old total.

The elephant in the room is the new adds. It's always a risk. One little bug introduced that the original inhabitants have never been exposed to can be a killer.

I had no idea live bearers had such a short lifespan.