I have less experience than you in this regard. I have grid power, so my off-grid systems are less critical. I found that I loose more power to self discharge steadily as they age and they will hold steadily lower voltages when left standing. The first batteries to fail were cheap batteries on a small 12V system, and then the first 48V batteries which were 'marine deep cycle' type. The golf cart type batteries I have still hold a charge but with reduced capacity. I am getting to know the routine. The capacity reduces as they age, and generally not linearly, but at an accelerating pace. You have much better quality batteries, but I suspect the trends will be the same. I think you will definitely accelerate the demise of the surviving half of the string because of the increased depth of discharge it will now have to endure each cycle.
When to replace probably depends upon how much failure is tolerable. If the power only went out every great once in a while, you might milk them along, but the situation will steadily get worse until you have to act to keep the family happy.
I don't know the conditions or life expectancy of your surviving batteries, but at present you have paid about $636 per year. If you were to pay $437.5 or a replacement battery and you got even one additional year after the purchase of the new battery from the bank as a whole you would be ahead of the game economically as compared to replacing the whole bank now, provided you could keep the two strings working. If you got one year out of running it on a half a string or two years more if you replace the one bad battery, you would still be ahead economically. Mixing old and new is certainly not ideal, but it might be the cheapest option in terms of long term costs given what you have to start with. Just my 2 cents, Rich