Partially - it's true that if a lot of people are on the job market it works in favour of the employers, but it's not as simple as that.
Your argument presupposes that everyone who left the banking business would seek employment in the same field, and that's not likely.
You're also presupposing that banks would be prepared to sit back and take the risk of key people leaving.... and in a competitive commercial world that's not likely either.
Bonuses in the banking world are a cultural thing, and what needs to change is the culture - the whole industry has to act at the same time in order to maintain the playing field as level as possible for all banks. That way, there would be little incentive for highly-paid people to leave one bank and join another.