Hi neo8175,
Welcome back! And thanks for the update on your configuration.
To answer your question, yes, it is normal for some softening to happen in the area where you sleep with any foam, even latex. The largest mechanical stress occurs in the uppermost layers of any mattress, and the softer foams will tend to be less durable than firmer foams. Latex changes less than other foams in loss of support factor, and Talalay Global rates their ILD loss in the 6%-10% range in some testing I’ve seen, with a compression set of 3%-4% in testing. I do not know the ILD of the foams used in these tests, but these are results that tend to be superior to most other foams. You may also wish to read post#2 here.
With your configuration, you’re using 4" of quite plush Talalay right now in the upper layers. I know that SleepEZ had mentioned in your earlier conversations that you could try mixing the order of the layers if the S/M/F turned out to be a bit too plush over time, and you may wish to try out the (top to bottom) M/S/F combination, with the 1" plush topper above that. Where you initially needed a bit more plushness when the mattress was new, now that the foams and covering have softened a bit (and maybe even you have changed a bit 
) it may be that you desire just a bit more surface firmness, so this rearranging may do the trick. It certainly would be worth a try, and is one of the great benefits of having a component-style latex system where you can rearrange and change layers over time.
Also, just because I can’t recall, what are you using underneath your mattress for a foundation and support?
I’ll be interested to learn of any experimentation you may with the layers.
Phoenix