I'm curious as to how many people actually keep 90% or more of their total liquid short-term assets in a bank. It seems like a basic rule of diversification of never keeping all your eggs in one basket, including the the bank.
Generally though there's things like Brokerage Accounts etc which offer higher levels of insurance (SIPC) and easy mechanisms to spread monies across several banks and account services (CDARS for CDs and MMAX for money market accounts).
At that point hopefully you have someone managing your money wisely to help navigate these kinds of things.
'Safe' stocks (utilities, etc), precious metals and real estate are all nominally safe investments as well.
If you would like to increase the interest received on your money, then holding a portfolio of value stocks or an index ETF will outperform your bank account by a large margin over time. This takes very little effort, but will suffer from the occasional (large) drawdown that you have to sit out or this strategy won't work.
If your primary objective is capital conservation then buying high quality debt of countries would be a good idea. Singapore or Switzerland are good candidates if you consider it a real possibility that the US could default on its obligations. Otherwise you could hold Treasury bills and roll them over once they mature or you could use an ETF like SHY that holds short-term Treasury bonds.
You could also buy precious metals if you are convinced that civilisation is going to end in the foreseeable future. Drawbacks are fluctuating prices and the complete lack of interest payments.
I'm unclear if you're asking how many people keep 90% or more of their total liquid short-term assets in a bank or not.
I'd expect most people keep most their short-term liquid assets in a bank since this is usually just their cash on hand. But this is not "all their eggs" considering their long-term non-liquid assets are probably tied up in some sort of retirement vehicles (which are hopefully diversified), and their homes.