There is a problem with your logic. You are going on generalities when every case is specific (and different).
For starters, if the position is extremely closed (i.e. a lot of locked up pawns) or if all the pawns are on one side of the board, then the Knight pair might very well out do the Bishop pair, even though it's an ending.
A single Bishop might dominate a single knight in an endgame. For example, ignoring other factors, like where the kings are, a White Bishop on c5 will completely dominate a Knight on c8 of the opposite color (or any other scenario of Knight on the edge, Bishop 3 squares away in the direction perpendicular to the end of the board).
On the flip side, put a Black Bishop on d8 and a White Knight on d5 with White pawns on b4, g3, and h4. The Bishop is completely dominated.
There are other factors as well. The old adage by Capablanca states "Queen and Knight are better than Queen and Bishop", and more often than not, that's true, but not always. Similar thought about Rook and Bishop being better than Rook and Knight.
Additional items include the fact that in order to gain the Bishop pair often takes time and multiple moves of a piece. This typically means a development advantage for the player without the Bishop pair. He should be trying to attack as quickly as possible. A concept known as Increasing the Speed of your Knights.
Another concept that baffles many is that a Bishop can very well control squares of the opposite color. Case in point, take the Nimzo-Indian, or the main line of the 2...Qxd5 and 3...Qa5 Scandinavian. In both of these cases, Black's Dark Squared Bishop is pinning and often eventually capturing a White Knight. After 1.d4 Nf6 2.c4 e6 3.Nc3 Bb4, Black's Dark Squared Bishop is actually controlling LIGHT SQUARES! Think about how a Knight operates. A knight sitting on a dark square controls light squares (as few as 2, as many as 8). A knight sitting on a light square controls dark squares. Therefore, Black's Dark-Squared Bishop is pinning White's Knight on c3, and because of that, the Knight has lost control of e4, and so White can't play 4.e4. The Dark Squared Bishop is controlling the Light e4-square!
What you need is the book "Bishop V Knight: The Verdict" by Steve Mayer, written in 1997. Most sites will say it's out of print, but last I saw, some of the specialty sites, like chesscafe.com, still have it in stock.
Some people believe that the knight is better than the bishop because of its unusual movement, while others like the bishops better than the knights because of their wide range, and some people think that both the bishop and the knight are equal in value. While this is true according to point value, it is almost never true in game play. The knight is special because it can jump over other pieces and can fork easily. The bishop is special because it is the only piece that can only move diagonally, and can easily pin. Let's talk about the beginning of the game, or the opening. In the opening, the knights are usually favorable, because they can jump over other pieces, and bishops are usually blocked by other pieces, specifically pawns. While this is usually the case, there can be some execptions, but this is usually how the opening works. You should not start to trade your knights for bishops that early in the game, as knights are a very valuable and powerful piece. In the endgame, bishops are usually favorable, as the long diagonals that they run on are usually open, allowing them to move freely. They especially help with pawn promotion, as they can quickly get from one side of the board to another. You should not trade a bishop for a knight in the endgame, because bishops can really help you in the endgame. Normally, two bishops is better than two knights. This is because two bishops together can control all of the pieces on the chess board, while two knights cannot. Also, two bishops can force a checkmate, and while it is possible to checkmate with two knights and a king, it is not possible to force checkmate, which is why two bishops generally is better than two knights. However, one knight is better than one bishop, because one bishop cannot control many squares on its own, and a knight has its special ability to jump over pieces, and in the endgame, it is much easier to fork pieces and win the game. So, there you have it. It ultimately depends on the position of your pieces to decide which piece is better, but if you have any additional comments, please feel free to comment in the comment section below. You can also look at the boards I have below. Thank you for taking your time to read this, and I hope this article has helped you. Thanks!