At a point, the curve begins to flatten out. At this point there is diminishing returns. The diminishing slope of the curve at this point reflects the law of diminishing returns. The law of diminishing returns says that once amount of capital per worker reaches a certain stage, the amount of additional output gained is less than the amount of extra capital being employed. The extra capital does not give greater output.
Technology can improve the quality of capital.
If capital improves, it makes the PF shift upward. There has been a decade to decade slowdown of growth of productivity since 1940. A slowdown in labor productivity usually means the standard of living also grows more slowly. Workers in the US are still the most productive in the world, but the slowdown has narrowed our lead. This may be true in a small part, but we have seen investment increase so this should have taken care of the capital deepening problem in the future.