So... this is bad how?
Expansion packs tend to sell for $30. Sometimes that is high, sometimes that is low (same with the $50 for a new game). You pay less because you have less content.
So why not the same for the core game?
Yes, it has room for "abuse" and the like (but so did expansion packs), but that is assuming you actually WANT everything.
Let's use Dawn of War 2 for example. The core game's SP is just Space Marines. The expansion is going to add Chaos.
But let's say they had even more planned. Pay a little more and you can play an Eldar or Orkz campaign. I would not really see a reason to play as space-elves, so I would never bother to pay for the Eldar. Orkz, I would probably grab immediately. Either way though, Relic would still have focused on SM during most of development, and then stated on Eldar and Orkz after the beta testing process really began (ie. game is pretty much done, just internal patching and tweaking).