Blimey, do you loons think Sainsbury's is subsidised if it sells bog roll cheaper than the corner shop?
Rather than having a different business model or overheads?
Sainsbury's may even be making more profit on each lower price bog roll.
(Could be any two businesses with vastly different overheads or model, so don't bother with any "Ah but Sainsbury's specifically..." squirmification.)
ProfPlum, your little piece of financial wizardry is pretty much the model which caused Southern Cross care homes, to close, leaving the industry scrabbling round to find new providers for 31,000 vulnerable residents. It's also what caused the failure of endowment mortgages, IIUC.
That's because those clever calculations fail to include or properly understand risk. Which, surprise surprise, is the same failing which caused the banking crash in the first place.
Anyone who approaches very, very long term financial matters (as housing is) with statements like "invest the money at, say, 3%" has so many red flags waving over them they look like the Red Army.