Fair enough. If we're talking about discretionary luxury spending, then you can take on a bit more risk to get to your desired income with a lower FU number, as long as you're willing to curtail spending if things go bad.
That said, it's a lot easier to increase spending than to decrease it, so I'd want that risk to be small, or to have a desirable fallback for added income.
Bottom line is once you buy those bond funds you should only be spending your coupons/distributions. There's a chance your distributions might decrease if for example the yield curve flattens or interest rates decline and the fund has to roll over into lower yielding paper, but for the purposes of discussing FU money you should probably be buffered to withstand a moderate decrease in monthly interest payments anyway. Ie, in practice you're not going to spend your entire monthly distribution, so any cash leftover can be rolled back into your investments and likely moderate any of the risk incurred by not going with a full 'risk-free' portfolio.