This is probably true, and it's one of the main reasons I don't like it, even though I've only just started dwarves and am not at orcs yet.
It is one thing to have a "free to play" game, where given time and effort the free to play players can reach a similar point to the pay to players. It is another thing to design the "free to play" game so that eventually you HAVE to spend money to progress. The players who don't pay exist to make the world more interesting for the people who do pay. So that the players who pay can contrast their slightly larger, more advanced city, with snails, and other paid advantages, with the slightly smaller, no paid advantages, no premium expansions, cities of the free to play population. Driving the free to play population out in the name of an act that tries to force you to buy snails can have a serious negative effect on the experience of the people who are willing to pay, because it takes the people they were paying to compete with out of the picture.
As an addition, I'm WILLING to buy diamonds to buy premium expansions because I like nice big cities, but I'm not willing to buy snails when I reach that point because I have seen both snails and ponds in other peoples' cities and the ponds are prettier.