Sparge temperature has little or nothing to do with tannin and silicate extraction. Note that you can boil decoctions for hours without a problem -- as long as the pH is okay. pH is the key to tannin and silicate problems. Runoff density is questionable. Homebrewers are told to stop runoff somewhere before reaching 3°-4°P to be safe, but professionals run off to near 1°P. The supposed mechanism by which density would play a part has never been proposed that I have seen, except in an offhand comment I once saw from Martin Brungard hypothesizing about some possible effect of osmotic pressure. Probably the assumption behind the homebrew rule is that pH will be too high at this point, but with no alkalinity in the sparge liquor this is unlikely, and the assumption must be that homebrewers neither can nor do monitor and control alkalinity.
Short story: Those enzymes in the mash are quite happy across a wide range of conditions (you definitely do need to be in that range, but is a range.) Sparge alkalinity (or lack of) is crucial, and kettle finings and yeast have very specific requirements.
The focus on exact mash pH, and especially the fantasy of some magic software to reliably predict it, is a homebrew thing. The professional literature and commercial practice focus on getting the correct conditions in the casting wort.
Again, there's been a lengthy thread or three on this over on the Brewing Science boards.