Your calculations are grossly inaccurate. You only pay taxes on realized gains, and for a 5% yield to become “probably more like 3%” it would require a 40% capital gains tax rate. Current LTCG rates are 0, 15 and 20%, and that upper 20% requires realized gains around half a million in one year. So the most likely interest rate for most posters on 5% will be… 5%. Some may see 4.25% - still a in. Remember, using investments to pay off a debt will also trigger capital gains tax, so the tax liability is likely more paying it off, not less.
Correct me if I'm wrong, but I believe that interest on money market accounts and CDs are taxed as income. Everybody's tax situation is different, but my marginal tax rate is around 35%, bringing a 5% yield down to 3.5%. If I was OP I'd need a fixed yield of 5.77% to just break even.
I bonds were recently yielding far north of that, but they adjust - with moderating inflation it's hard to imagine anybody having locked in hundreds of thousands of dollars of high yield i bonds.
Are you suggesting investing in bonds to cover a mortgage? VGLT is currently yielding 4.04%, VFITX less than that. After taxes that doesn't cover OP's rate either (assuming a 24% long term cap gains rate using myself as a proxy).
I just don't see what reasonably safe asset is available to a person with a ~35% marginal rate and a 25% long term rate that will provide more than 4% of yield post taxes. I suppose OP could fund a mega backdoor roth (if available) and hold fixed yield investments that will have zero tax liability, as long as they're committed to several years of contributions to get all their money into the roth. However, that's not available to most people.
The real power though is having more liquidity. When things go wrong having an extra quarter million (or much more over time) is much safer than having lower monthly expensives of about ~$1,150
That depends a lot on individual circumstance and one's personal definition of "safety". Carrying a mortgage when living off a portfolio is
riskier than paying the loan off. For some "safety" might be locking up their assets in an illiquid asset that they cannot be tempted to spend. For others, "safety" might mean the smallest possible risk of losing net worth. For others it means the greatest safe withdrawal rate of their portfolio. And for yet others it might mean having the greatest amount of liquid assets on hand at any given time.
None of those are invalid perspectives. I don't see a one size fits all argument.