Hydrolysis water is still coming from the mash, so mass being equal means unless volume is also reduced byt 3.5-4.0% the change in gravity will be smaller than that or possibly even zero if volume is unchanged. Besides that any method relying on changing gravity or even refraction index (the latter does change as hydrolysis proceeds even with invariant gravity) will be totally swamped with noise and be as good as useless. For example, how do you distinguish between gravity increasing by 1 point because more starch/dextrine has come loose from its matrix and finally gone into solution (noise) from the same change occurring simply through glycolysis alone (signal)? You just can't as both scenarios are indistinguishable. In the worst case if enzymes are deactivated prematurely you will eventually observe a stable gravity (or refractometer reading) as no more extract goes into solution and/or is converted, but you will never achieve full conversion no matter how long you wait. Your simple test has only fooled you into believing you have achieved full conversion but you never have.
With the iodine test you directly assess the composition of extract and have a clear pass/fail answer, taking all of 30 seconds at most. Anything else is no different than mashing for a set duration and then hoping everything went well, with no direct confirmation.
The formula for Plato is:
°P = 100° * Weight of Extract / Weight of Liquid = 100° * Weight of Extract / (Weight of Extract + Weight of Water)
The denominator does not change with saccharification, as you state, but the numerator gets larger, so °P goes up.
Monitoring SG during the mash is basically monitoring the increase in dissolved solids. SG will increase until there is no more solid material going into the mash. If as I stated in my previous post, saccharification proceeds much faster than starch dissolution, then once dissolution has completed, saccharification will be completed in just a few minutes after that. And since all the extract is in solution, along with the enzymes, saccharification will continue during lautering and heat up to a boil (until all enzymes are denatured), or continue during heat up to mash out temps (if mashing out prior to lautering.)
The iodine test is only sensitive to amylose chains over a certain length. Once you have no long chain amylose in solution, the iodine test goes negative. That still doesn't mean that saccharification is complete, as there is more molecular weight reduction still to occur.
Many brewers report having trouble getting a negative iodine test if they have grits in their sample, so they carefully exclude grits from their sample, and get the desired negative results. The implications of this is that there is still undissolved starch in the grits, but that the amylose in solution got hydrolyzed below the detection point before gelatinization and dissolution were complete. More evidence that saccharification proceeds faster than gelatinization and dissolution. And if they still have undissolved starch in the grits, they will not have 100% conversion.
Since SG monitoring directly measures completion of dissolution it is a more reliable test for determining when the mash is "almost" complete, and most likely will be complete before the temp is raised enough to complete denaturization.
If your enzymes are denatured prior to the completion of dissolution and saccharification, it doesn't matter what test you use, further saccharification is ended. It's really up the brewer to insure that their grain bill has sufficient diastatic power to complete saccharification, and that they don't denature the enzymes prematurely by mismanaging mash temps.
Brew on