In general, your reasoning has some basis in evolutionary psychology. It is incorrect, however, to imagine that evolution is about "the survival of the fittest." In reality, it's about survival of the best-adapted. Although clearly "fitness" in a personal sense is important to individuals, for a species as a whole it is about the degree to which it is adapted to its ecosystem. Darwin understood this, which is why he never used the phrase "survival of the fittest" because it's very misleading.

In a lifetime spent working, living, and traveling all over the world it's become apparent to me that we respond unconsciously to environmental cues. In countries that are somewhat unsafe, women tend to prefer men who are strong enough to deter would-be aggressors; in comfortable safe Western nations however, females prefer more androgenous males who won't pose too much of a threat. While all women respond (albeit mostly unconsciously) to overt displays of wealth, this response is stronger in societies that are generally impoverished and less strong in societies where wealth is more evenly distributed. We are all just simple primates, after all.

Therefore is is obvious that successful older men will leverage their various assets to attract younger women in need of their resources, and that younger women will respond favorably to being so wooed. Whether this is the basis of a lasting relationship, however, is entirely up to the predisposition of the people concerned.