I agree with this.
The six clubs you have mentioned are a cut above the rest in Scotland, in terms of their support. I reckon Dundee Utd are averaging more than most SPL clubs playing in the second tier without Old Firm crowds - though I acknowledge people like to see their side win and so that's drawing in the crowds at the moment, too.
Motherwell, ourselves, Falkirk, Dundee, Partick Thistle and Kilmarnock all have a similar support, and with the exception of Dundee, the proximity to the arsecheeks will always mean we struggle to attract big crowds. In fact I'm sure I recall reading how Kilmarnock's average crowd was only around 7,000 when they won the league in the 1960s. As you say, we can do community work and try to attract new fans, but we need to accept that realistically with our resources it really is a case of trying to be the best of the rest.
Having said all that, Motherwell and Kilmarnock have punched above their weight for a good while. Conversely Falkirk and Partick have both underachieved, IMHO.