Once MS Always MS

It really depends exactly what happens to it. Its not necessarily always going to be perfect, but just because it tones doesn't mean it wont be anymore. It will always have a great strike though. For what its worth I've seen some in the old holders from probably late 90s time frame that still look as good as the day they left the mint.

The slabs arent 100 percent full proof but they do provide a good amount of protection, just dont leave them on the dash board of your car and expect the coin not to change lol

Those shops and shows also have massive amounts of coins graded, you basically know the percentage the are going to come back 70s when your submitting thousands at a time. For modern coins the striking really is that good now that you could probably argue that more would qualify for a 70. Go look at some of the 69s and then try and find the imperfection that kept it from being the 70s. I disagree that more people buy labels than coins. Some people do buy the label but most buy coins that they think fit the label. The first strike and stuff like that is marketing since the TPGs do need to make money, but some of the labels do look cool. I couldnt care less about the first strike but I love the PCGS flag labels for the State Quarters and think it adds to the overall visual appeal.

Id buy that maybe ANACS helps out HSN with boarderline 69 70s getting 70s. But they arent going to just fill quotas to fill quotas as that would ruin their reputation and put them out of business. NGC and ANACS are a little loser with their 70s than PCGS, some may say they over grade 70s and others would say that PCGS under grades 70s. Which one is true, probably a combination of both.

You are right though that if you tried to cross 10 70s they wouldnt all be 70s but some of that is human nature. They all have the mindset that their the best and will be looking for any excuse to not cross them all to prove their grading is better

You also have to understand grading is subjective and is just an opinion, done to written standards, but just an opinion. Unless you consider toning an "imperfection" what, except physical or environmental abuse of the slab, would do anything to change the grade.
As far as HSN goes, when they submit 4000 coins and say I need 1000 MS70's, yes you don't keep your largest clients by continually disappointing them. But also consider, when you grade one coin it is easy to pick out flaws and grade as a one off. When you grade a thousand of the same coin, it is only natural that you tend to grade. them against each other, instead of an unseen ideal.
If one coin is an MS69 then every coin you see nicer is automatically an MS 70. It is not feasible to have a thousand people grade one or two coins each. I think this goes a long way in explaining why.