Some interesting UK research (although the caveats are important):
http://paullewismoney.blogspot.ie/2016/06/cash-beat-shares-from-1995-to-2015.html
http://paullewismoney.blogspot.ie/2016/06/cash-beat-shares-from-1995-to-2015.html
And in any event interest rates are zero or close to zero now and should stay that way for quite a while.
Maybe that tells us something about expected returns on equities.
Cash is not where you want to be over the next 5/10/20 years.
How could you possibly know that?
Fixed income investments have certainly beaten equities over 5/10/20 year periods in all developed markets in the past.
Prospective returns are primarily a function of how assets are priced right now. On that basis, I wouldn't invest in fixed income with my worst enemy's money. Meanwhile, European equities are trading at what, 15/16 times? More or less average basically. And it's stated policy to keep interest rates behind inflation for the forseeable future. That is not an environment in which one wants to be in cash.
Nobody can predict the future, but based on current valuations, how can fixed income outperform equities over the medium to long term?