A &quot;purely mathematical standpoint&quot; is meaningless without context.  You&#x27;re arguing that paying money for something you need is a bad investment.  Paying money for potatoes that I&#x27;m going to eat is a bad investment, but I don&#x27;t have a choice.  I have to pay for potatoes and I have to pay for housing.<p>Even your edit, which mentions rent, pretends as if the interest paid and the rent always end up equal.  This is clearly not the case.  Sometimes it makes more sense to buy, from a &quot;purely mathematical standpoint.&quot;  The benefit of mobility has a different worth to different people.