Ok fair enough if the probe fails and you are LOP at that time I agree you where compliant when it was set so that's fine, but what of the next flight ie returning to base let's say it's a two hour flight. Surely you can't comply then with a failed probe, your only option is to run ROP to be compliant.
How will you know the cylinder is running ROP if the EGT probe has failed?
Think about what Jabba said. All your EGT probes are working. You set a mixture that results in all the cylinders being LOP. You have a known MP, RPM, density altitude, IAS fuel flow and CHT for each cylinder.
You then snip the connections to all the EGT probes. What changes in the engine? Nothing.
You then do a touch and go, and climb at full rich (not the most efficient way to get there, but let's not complicate things) and max RPM, and then level off at the same density altitude as before and set the same RPM and fuel flow as before. The MP is the same as before, and you note the IAS is the same as before and the CHT for each cylinder is the same as before.
It's very (very) probable that all the cylinders are at the same or nearly the same point LOP as they were before. And anything that would change that outcome would mean you couldn't be sure you were ROP and "compliant".