In their time they were....different era. They faced the same pitcher the whole game and didn't see 100+
The speed of athletes today put those guys to shame. Different time, that's all. Imagine if Griffey, Trout, Bonds, and many others played back then
Until the mound was lowered after 1968, that was the era with the most advantages to pitchers since before World War I. Higher mound, no thin handled bats, no body armor, different balls, parks generally larger, no trips to Colorado or Arlington, regular double headers, etc.
The absolute best, and virtually only way, to measure baseball success is in relation to their peers. Mays and Mantle were so far higher above their peers than Griffey, that there is no way any era adjustment could make up for it. They did so far more to help the Yankees and Giants win, than Girffey did to help the Mariners and Reds win. Sports talent does generally increase from generation to generation. But if the 40 years difference is enough to make up for how much better Mays and Mantle were, that means this next generation may have up to 15 center fielders who will be better than Griffey.