This is my 686-4 that has a groove worn in right above the forcing cone. Looks like after a few thousand more rounds the frame will break in two.

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That happens after a certain amount of shooting. Gas from the fired shell is "flame cutting" the top strap. The experts should be along to explain in more detail. It looks like it won't get much deeper. I have the same flame cut in my model 19. It seems to cut so far and then stop.

Flame cutting is a destructive phenomenon that occurs in some firearms (usually magnum revolvers) as a result of hot gases under high pressure.
The most common manifestation is on the underside of a revolver top strap, where the hot, high-velocity gases released when a round is fired begin to cut through just above and in front of the cylinder. In most cases, this is self-limiting; eventually, the depth of the cut will place the undamaged material far enough away that the heat and pressure of the gas will have dissipated to harmless levels. This limiting tends to occur before structural integrity is compromised.

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Shooting light weight magnum rounds like 110-125 grain are more prone to have this deeper.

There is still a good life time of shooting in that gun. When it gets to about 1/8 of the top strap thickness, then you are going to have to worry about the top strap cracking.