I think the problem lies in who they payed, not just the money they payed. Heyward will probably be worth most of the money he gets, because of his age (unless somebody overpays for his defense, which will slide, and his offense has never been that great). Bryce Harper will almost certainly be worth what he gets payed, because of his age (unless, of course, he gets hurt). The same to Giancarlo Stanton.
Giving ten years to A-Rod at 32? Ten to Cano at age 30? Bad idea.
As for diminishing returns, well, Los Angeles ate a lot of money in trades for role players, and a lot on gambles, see Morse and Latos. They just don't care, money means nothing to them so they will just throw it at anybody. Their main problem is in who they evaluated, if they have infinite money why don't they just give money to Cincinnati for Chapman and Cueto? Why try to be clever with Alex Wood and Luis Avilan?
But of course, the playoffs are a crapshoot. Greinke and Kershaw were undoubtedly two of the three best pitchers this year. If you're a man of science, you pick them every time you're in a playoff. Neither of them were excellent this postseason; they were quality, they kept you in the game, but that's a whole lot less than what they did for almost every start of theirs in the regular season. And the other pitcher? He just got shelled by the Mets, after getting shelled by the Cardinals, after pitching a CGSO against the Pirates.
So the better question is, why overpay for an "ace" to lead you in the postseason when that's so unpredictable? Just put together a team that's solid in the postseason, players who can "get it done," and hope your players execute, whatever that means. But is that team good enough to get you through the grind of 162 games?
So then, the real reason you pay those players, is just to get you there, which, as the story shows, has happened the vast majority of the time. Only three times out of fourteen did they miss. Why are guys who are good enough to get you through 162 falter when it comes down to eleven? What's the correlation, how do you find balance? I don't know, ask San Francisco, I've done enough thinking for one post.