This doesn't take into account available volume. For that you need to solve a min-cost flow algorithm, using for instance the Ford–Fulkerson algorithm.
Low volume is one of the reasons that arbitrage was so difficult with Bitcoin. While an individual could be somewhat profitable, it just couldn't provide adequate returns for a medium or large organization.
Available volume? Forex is the most liquid market in the world.
1) There's still finite volume at a given price, and the arbitrages, when they do happen, typically involve small volumes.
2) There is more to arbitrage than Forex markets
Yes. But if your arbitrage pattern is only 3 pips profitable, and you are exchanging millions at once, you might expect that the amount of available orders matching your path may actually matter.