Not sure if you're saying that it's rotting because the fruit have been left hanging too long and have become overripe, or if you're thinking there's some pest or disease. Either way, It does sound like your trees would benefit from some thinning though. You can easily thin half (or even more) fruit from plums and peaches. You'll be pleased to see that the remaining fruit will become much larger. It also will put less strain on overloaded branches. I wouldn't remove all the fruit unless you're REALLY in a hurry to get the tree to a bigger size. After all, the fruit is why you're growing the tree, right? Unless, that is, the fruit is ripe, in which case you definitely should pick it all, for obivous reasons. Even if you can't consume it all, it's better not to leave old fruit hanging, because it can harbor pests.