if you never have to borrow money for emergencies, your investments are too liquid
That assumes there is price for liquidity which you are paying. I am not sure this is the case for most normal people (as opposed to, say, those who invest into private equity) now because other than real estate most other available investments are quite liquid.
Essentially, most of people's investments are bank accounts and market securities (again, real estate is the big exception). Liquidity shouldn't be an issue here.
For recent college graduates, their best investment opportunity is early repayment of their student loans. It's essentially guaranteed 4-5% return (whatever their loan rate happens to be). Note that this "investment" is completely illiquid.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.