Inflation is the short answer, yes. There's a bit more to it though.
The more detailed answer is that "money", whether it's physical bills/coins, bitcoins, or digits in an account on a computer, is just a proxy for real things - goods and services. Direct barter is pretty inefficient, in terms of time/effort/etc, so we abstract it with money. Now, while there's really no upper or lower bound on how much 'money' there is, there's a finite amount of physical goods and other productivity in the economy at a given time. Ideally, we'd have a perfect 1:1 ratio so that the amount of money flowing around matches the amount of physical goods/etc. In practice it's pretty difficult to actually do that to an exacting amount, so keeping it reasonably balanced is one of the primary responsibilities of a country's central bank, like the Fed.
The economy is generally growing, which means more goods and services, which means more money is needed to keep pace. Inflation isn't inherently bad, not in small amounts. It's only when inflation goes high that it gets bad. More importantly, negative inflation (deflation) is really really bad, because in that situation, the economy grinds to a halt because nobody wants to spend money (because it'll be worth more tomorrow), and we get into a nasty cycle that's hard to break out of - one that usually requires a lot of inflationary pressure to counteract, such as printing money or a central bank injecting more funds like the Fed did. Otherwise, if the government isn't willing to do enough, you wind up like Japan with your economy stuck in neutral for a decade or two.
So back to the question of reimbursement for bank theft losses - sure, you could probably absorb one or two of these without any real economic impact. The problem tends to come in the long run when you've established a policy of doing so, because it can quickly get out of control - try explaining why you'll reimburse Alice but not Bob for their losses.