But they should have that kind of supporter base. They've been in the comp for nearly 60 years.
Penrith was tiny when they were admitted in the competition. In fact the estimated population of Penrith City at the time was just 12,138, whilst the Blue Mountains had 21,316.
In the 1990s, when they reached their first premiership, it finally got to around 100,000 for the Penrith region.
By early 2000s, Penrith's population rocketed to 171,000 and now it is around 250,000 (and of course all the surrounding areas have exploded as well.)
The population profile also started changing by around mid-90s, from people migrating out there, to expansion from within (which is generally what you need to build a supporter base)
Now I'm not sure what criteria they used to help decide what teams to add in 1967, but whoever decided on Penrith either had incredible long term foresight around the growth of Sydney or simply didn't care about the size of the community underpinning teams.
As others have said, over the next 10-20 years, the club is going to become one of the big clubs. The success + local population growth + strength of RL in the region almost guarantee it.