I haven't read the original paper, but my assumption is that if their model suggests the universe is much older and that things like light speed actually decay over distance, then maybe they are suggesting that some of the other constants of the universe are also decaying leading to the different observed rotations. But, I'm not a physicist.

Speed of light decay should be fairly easy to disprove.

Anything that contradicts Special Relativity requires an exceedingly compelling argument or preponderance of evidence in favor, and Tired Light does not have it.

I'm not a physicist, but, why should this be fairly easy to disprove? The person proposing this theory seems relatively informed and obviously still published, so it's obviously not trivial.

My (naive) assumption reading the article was that he wasn't proposing that a photon loses velocity over long distances, but that the value C was not constant on a multi-billion year time horizon. I don't see an easy way to prove that it is not changing if the rate of change is functionally unobservable.

But, maybe I am way too uniformed to speculate on this.

But we observe this everywhere, even in the Andromeda galaxy, so, universe age is not a factor for the rotation curve of galaxies.

You gather more evidence. If the evidence you need is out of reach then you have identified a more immediate problem to solve.

Quantized Inertia is a new theory that works and is interesting. Although, not rigorous and is speculative at best