That's correct, mark. The key to copyright infringement and why it's illegal is the following:

Suppose I create the Enterprise from Star Trek down to the last detail and make a game from it. Let me say that it's a fan game and I offer it for free. The game SUXS! (like on the level of "legacy" ) I mean royally! Now tons of people who play the game are going to say "how could Star Trek have sunk so low? I'm never going to buy anything from Star Trek again!". Now mind you, ST had nothing to do with the game but it's reputation is suffering and all because someone created a product that was identified with the original trademark without their permission.

Hence, as long as your product does not refer back to anything else, you are fine. For example, you could put ST like ship engines on a Tie Fighter and call it "The flying pig" and that is perfectly legit because no one in their right mind would think that the model is referring to either the SW or ST franchise. On the other hand, if you call it "The Tie'd Enterprise"...

In general it's pretty easy to check yourself and you hit upon it: if you are using other copyright for inspiration but create your own work AND it doesnt refer back to the original in any way, you are pretty secure. I see pretty secure because if ST or SW wanted to go after you for "The flying pig", they are free to do so... but then the burden is on them to prove that you were infringing and it's unlikely to happen.

This is why whenever another Stargate clone, Zelda tribute, or Mario Model comes up, I warn people about it because they may not get in trouble for it now, but if they don't know about copyright law and get it in their heads that it's okay to use Mario for a Fan Game, they are setting themselves up for trouble (or at least lost work) down the line.

Will Nintendo or Universal or Paramount come down on a 3DGS user for making a fan game?
Unlikely.
Is it better to be original from day one?
Absolutely!

(BTW Why, it doesn't have to be an exact duplicate. If I take mario and rename him Jario and paint him blue and set him to save the princess on top of a rhino named Moshi, it is obvious who you are ripping off and you could be in trouble. In general it has to be enough to recognize that the majority of the work came from another copyright for it to be infringement)