But who knows, when one day someone will come up with a better model that does away with the "dark matter kludge", it will turn out that these phenomena actually have a common root cause?
Besides, epistemologically it is very good to suspect and investigate the effects of DM on all kinds of phenomena. Then, if it does not exist, we will have many more points of reference from which to derive experimental or theoretical contradictions.
It does not really matter if those exist or are an artifact of current theories, replacing relativity is not that easy and there are already a ton of physicists working on that anyway.
In the meanwhile of either having instruments capable of detecting these or a new theory that demostrates that those are artifact emerge... they have to exist. That's the point of them.
It's astronomy. It means "anything that is not shining like a star but we can detect it via inference." Light matter is stars. Dark matter can include MACHOS: neutron stars, black holes, brown dwarfs, rogue planets. No new physics required for MACHOs, as opposed to WIMPs.