I believe that retroclones of old school RPGs from the 1970s and 1980s should be flat-out legal. If someone has an old dog-eared book from a basement and there is a cool mechanic or idea from it, I think they should be able to use it in their new game as-is.
These are two different things, and the fact that you conflate them shows that you don't understand the laws or the argument being had in this thread.
First, the Courts have ruled that "game mechanics" cannot be trademarked, and morons have misunderstood what that means for years. A "game mechanic" is merely an isolated function, such as
"roll d20 and add a modifier," or
"gain experience to level up." A COLLECTION of game mechanics (typically refered to as "a system") is an artistic expression and constitutes a common law trademark under US code. Wizards' OGL is nothing more than a trademark license, allowing the public to use their "common law trademarks."
Again, it is the EXPRESSION of game mechanics that creates the trademark. This includes, but isn't limited to: the specific names and configuration of attributes, the specific names and configuration of skills, names of feats (including the term "feat" itself), names and descriptions of monsters, etc. Any company has the right to withhold trademarks they don't want to include in a license, which is why the OGL restricts access to beholders, carrian crawlers, yuan-ti, and a few other iconic monsters.
If there is an individual mechanic from an old RPG, you don't need any permission to use it in your own product. A "retroclone," by definition, is not just a single borrowed mechanic, but the entire expression of mechanics. When you so ignorantly say it
"should be flat-out legal" to use these old systems, you are saying it should be legal to steal trademarks.
There are lots of variants of old games and even complete retro-clones that I consider to be genuinely positive development of the original. The authors shouldn't be considered evil plagiarizers, but rather trying to positively develop the games. To get more specific, I think anyone should be able to create their own version of Star Frontiers, even though the original is still available for sale.
But retroclones are already "legal" provided the creator follows the OGL. I read recently that a judge has even ruled that the OGL cannot be revoked and now must exist in perpetuity due to the shere amount of content created while using it. Wizards' has unbottled the genie, and they cannot put it back no matter how badly they want to.
As for your last statement, anybody CAN make their own version of Star Frontiers, they just cannot use any trademarks not expressly allowed by OGL /SRD. Basically, this means they can't call it "Star Frontiers", use any proper names for races, planets, or NPCs, etc. But there's absolutely nothing stopping someone from making "D&D in space." Plenty of people already have.
If what you are actually saying is you think people should be allowed to publish their own games using the Star Frontiers IP, again, you are advocating for trademark theft.