"We have concrete evidence that either a) a new type of matter and energy exists, or b) our theories need to be modified in some way."

a new type of matter is a modification to our theories

"Until we either detect dark matter/energy, or develop a theory that accurately predicts the behaviour we're attributing to dark matter we cannot say one way or the other which is the correct approach."

"We" the general public isn't in the business of saying one way or the other is the correct approach, and scientists aren't, either. Scientists conduct experiments and propose theories in whatever lines of inquiry interest them, subject to the constraints of getting somebody to pay for it. Many scientists have been interested in refining the theory of Dark Matter and subjecting those refinements to experimental tests, partly because the theory has withstood and only grown stronger by those refinements and tests. That's a success by any measure, and that success is partly why public funding agencies have been willing to pay for it. Like anybody else, they try to pick winners.

It could also be that we are not accurately modelling EM/SR/GR effects at a large scale, such as how they are warped by the different stars orbiting the arms of the galaxies. Or that when we extend QED/QCD to accelerating reference frames (general relativity) that dark matter won't be needed, just like how QED was formulated by extending electromagnetism/QM to special relativity (non-accelerating reference frames).

It could be. Anything's possible.