The problem with Pentax is that there isn't as much room to grow. They don't offer a full frame option and the lens selection isn't as vast as Canon and Nikon. They are a good value for the money, but there is a trade-off.
You're absolutely right.
I still bought into the Pentax system knowing fully well that I won't be able to upgrade to FF (for now). Going FF isn't something I think I would need. Who knows. May be the new Pentax-Ricoh relationship will produce some interesting products.
Pentax has this intangible appeal in my opinion. Their "limited" lens designs and the "odd" focal lengths (31mm, 43mm, 40mm, 55mm, 70mm, 77mm) make them pretty unique. But you certainly won't get the large selection of lenses and accessories like Nikon/Canon.
Their silent/fast focussing lenses (SDM/DC) aren't as refined as Canon and Nikon. And Pentax doesn't have fast affordable primes with the closest thing being 35mm f2.4.. On the other hand, their limited lenses exudes craftsmanship and care.
I see your point,
but (imo

) saying that there is no room to grow isn't universally true, it really depends what you're after.
The OP hasn't specified, but for example - plenty of users don't move much far beyond the standard kit.
Pentax offer (imo) more then enough for these users, and those who need more.
In the standard zoom range, Pentax actually have quite a decent selection for many levels of photographer:
DA 18-55 II/L/WR kit, DA 16-45mm f/4.0, DA 18-135mm WR, DA 17-70mm SDM, DA* 16-50mm f/2.8
+ the raft of forwards compatible FA and F zooms that both autofocus and meter on digital bodies.
As runlsd mentions, there's an interesting selection of unique DA and FA Limited primes for another target audience, not found anywhere else.
It's a different story with long fast telephoto glass, but with this widespread notion of "Pentax don't do high end stuff" trickling down and influencing the buying decisions of many people who are otherwise fine at entry level (and
more than that), it's no wonder Pentax isn't in great shape.
It's great marketing that is doing wonders for Canikon ("What camera can I buy next?"), but a
lot of people (imo) are ending up misinformed of Pentax's strengths. A lot.
I'll agree their DA* lineup leaves much to be desired, until they sort out their flakey SDM motors, I couldn't recommend DA* lenses for the moment. But they're
almost there.
(Imo) Pentax cator from entry level all the way up, spanning many flavours of advanced enthusiast.
If you're operating a business Canon or Nikon makes sense for the support networks, industry abundance/availability etc etc.
These ideas filter down the chain and help sell entry level bodies.
Then again, if you've done your research and find the Nikon or Canon roadmap attractive for your needs, then you've done the right thing, and there are
many fantastic options available there.
My point is, saying that there's no room to grow may be true if you want to shoot Wimbledon for publication, but otherwise, the line of where you can and can't go isn't always clear -
and no line fits all.