I just realized the reason that it seems like every philosophical idea is false or useless. When a philosopher comes up with an idea that’s true and useful, it gets picked up by humanity at large, people forget who came up with it, and it’s no longer considered philosophy.
Here are some things that philosophers invented, that are true and useful, and are no longer considered philosophy: systematic reasoning (the Greeks), the scientific method (Bacon), democracy (the Greeks), communism (Marx), much of basic music theory (Pythagoras), the concept of human rights (Paine, Mill, Hegel), and formal logic (Frege, Russell). We can see that most of these true and useful results are in the areas of epistemology, political theory, and ethics. It is interesting to note that I can think of nothing true and useful that has come out of metaphysics or ontology.
Philosophy seems to have a problem similar to the one that artificial intelligence has. In AI, whenever an accomplishment is made, people redefine the concept of “AI” so that what was just accomplished is no longer AI. Similarly, whenever philosophy accomplishes something true and useful, it ceases to be considered philosophy. This creates the illusion that philosophy has had no significant accomplishments.
That said, it seems to me that the vast majority of philosophical ideas fail to meet the “true and useful” criterion. Philosophy’s successes, while powerful, seem to be quite rare. This is perhaps due to the nature of philosophy. Since philosophy addresses such large problems, (a) it is very hard to make progress on the problems, and (b) when any progress is made, it is a fairly significant event. So perhaps philosophy has a bit of a “strike out or home run” dynamic, where the overwhelming majority of thinkers do not hit any home runs.