I am not sure what kind of companies are the "slog it out" type. Which company can be profitable (I assume) and not grow much in 2 decades? If they are just cash cows, why can't the VC/entrepreneurs figure out how to reinvest it in other sectors?
It sounds like he means that they're profitable enough to keep surviving and growing somewhat but not to the point where someone wants to acquire the business.
Due the emotional investment in the business the entrepreneur slogs it out hoping they'll stumble upon something that will give them the growth curve they need to get the acquisition / IPO / other exit opportunity.
I'm in the photography space and I know of companies who raised rounds years ago (2006-09). They are still around but it's clear they won't be a huge win for the investors.