most new businesses in the US are still bootstrapped, usually through a combination of family (non-)wealth, non-professional investment, (small) bank loans, supplier credit, and plain ol' hard labor. most of those will be small businesses, but many will grow to mid-sized businesses. most will not be software businesses.
software doesn't have an investment problem. in fact, it's a maturing industry where all the big money that investors can squeeze out has mostly been squeezed out. that's where the lack of interest in the sector lies, not some missing middle investment. investors are looking for bigger opportunities because the risks have risen, but there's such a glut of money looking for return that we have risky sideshows like crypto/nft's seeing billions pouring into it irrationally just because it could be big.
what is actually happening in relation to the middle is the hollowing out of the real middle class, where family wealth and non-professional investment is going to zero and becoming untenable for starting a new business, and economic rents overwhelm even those where it's available. the problem is that we're becoming feudal, and fewer people managing bigger pots of money is less efficient and less dynamic.