The main reason I ever get my coins graded has much more to do with ease of storage and preservation than caring about their investment potential, as I do not in any meaningful way consider my collection an investment (on the rare occasion I sell any of my coins, it's usually for the sake of buying other coins). But I like comparing them to see if I can see why the graded what they did, and not only do I find it almost impossible to tell a 69 from a 70, I can barely tell the difference between a 68 and a 69 sometimes. I've always believed that if you can't tell the difference between a 69 and a 70, you shouldn't pay for it either lol... and 70's just seem like a sucker's bet of an investment for modern coins when the majority (of proofs/commems) grade 69 anyway and 70's aren't even especially rare. One can make money in the short term separating fools from their money but in the long term modern 70's don't seem to be any better investment than beanie babies; the fad has to die some time. Maybe I'm just being cynical but nobody's ever gone broke by being too cautious. I've always hated when perception can somehow trump reality (why I don't like investing in a lot of stocks lol...) but in the long run reality tends to win out.
I prefer to think dumb and dumber. My experience has been that the MS/PR70 grade is an award of submissions. Nothing more and nothing less. In other words, submit enough Modern Bullion Coins and you'll eventually end up with a 70 that some sucker will pay moon money for. The exception is the Eisenhower Proof Dollar since they gave up on awarding these many years ago as the prices are simply out of hand and the TPG's would have to cover those prices with Grade Guarantees.