Central banks already ensure that if multiple people the money is promised to want to make withdrawals at the same time, they can all have the cash. But if a m

Central banks already ensure that if multiple people the money is promised to want to make withdrawals at the same time, they can all have the cash. But if a mortgage requires people to lock away funds for quarter of a century (because the bank can only loan out the funds if someone has the money tied up for that long) then whoever's lending is [i] diverting the funds away from more productive activity [ii] going to expect a very high interest rate because they don't see their money for such a long time and [iii] likely to be richer on average than the current beneficiaries of banking activity. Reducing transfer from productive to unproductive activity and the scale of transfers from poor to rich are desirable: maturity transformation is therefore a feature of the system, not a bug [n.b. if you force banks to borrow long and lend short you've eliminated the fractional reserve mechanism anyway]