I work for a company which
does pursue that market (we have everything from the sole freelancer to companies with thousands of workers), and I disagree.
Regarding your example, most of our clients with 2-5 users do ask us to set up different permissions. In fact, I'd say that worrying about that is much more common in small companies, where the manager is often the owner and therefore often hates losing control of the processes.
In terms of database items, the correlation does tend to hold, but the one thing about SMBs is that they vary a lot. If you ignore all those edge cases, you end up targeting just a small subset of the whole market.
And the last point is crucial. You write about the "needs of big companies" and "needs of small companies", but in general there's no such thing. Each company, large or small, has its own needs, and a cookie-cutter solution - even if generally targeted to "small companies" - is simply insufficient for many. But while a large company can pay well to accommodate its idiosyncrasies, a small company can't, because those needs don't shrink proportionally to the budget.
But I wouldn't say the market isn't being served, and the current bet on SaaS is a clear bet on SMBs, which often can't afford to pay for the purchase and maintenance of servers and the sysadmins to manage them.