There are three different factors, you cite TC:
-helmets - That was a necessity in cricket for batsmen's protection, and should have been implemented as soon as the helmets were invented.
-no bouncer (one bouncer) rule - Yeah I totally agree, this has made things much easier for guys bad at playing the short pitched delivery, ganguly, bevan, twose etc. I am not sure if there is a relation between this and ball tampering, but as you say, these rules have helped the batsmen and allowing ball tampering would help the bowlers so it would even things out a bit. Makes sense.
-bat technology - I guess this has helped as well, but I still think this cannot be compared with ball tampering. Advances in ball technology can be compared with advances/changes in ball technology (introduction of white ball etc) only, I think.
And also, I think the Derek Pringle example is rare
Derek Pringle had loads more ability compared with Chris Pringle, whom I was talking about. Chris Pringle did nothing of note in the series and his career, only in that match he was absolutely unplayable. But he did use a bottle cap, which you say should be disallowed.
Which brings me to the question that I earlier asked, if its totally legal to do seampicking/nail scartching, would guys be able to grow nails sharp enough to do enough damage to the ball as would be possible with a bottle cap?
Every fielder working on that ball with his nails, diggin hard into the ball, throughout the innings, would do considerable damage I think.
If the answer is yes, then I think we would have a big problem on our hands. In any case, its a suggestion worth trying, maybe in the county championships or some junior league type matches to gauge its impact in real life.