Why wouldn't it be sustainable? This is not just moving money between Chase, Wells and BofA. There are an effectively endless number of banks. And most simply put a 12-month restriction on you clearing a new bonus with them.
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once I build up some cash reserves
In addition to cash reserves, I'm using them instead of bonds in my portfolio. I invest in bond funds for the sub-6% returns, with variance and risk, to diversify away from stocks. Well if I can earn even more with no variance and effectively no risk -- that seems like a better idea.