If it is not changeless than it's not eternal.
In more general terms in the argument, the first cause is independent (in all senses of the word); everything we know in the universe is dependent
That's a hypothetical, free wiling assumption to "balance out" the equation. That's no different than a cosmologist saying there is "dark matter", there has to be, even though we don't know what it is or how to recognize it! But we need it to balance out our equation!
Guess what? maybe the equation is flawed!
That's what philosophers and scientists engage in continually. Until they come up with something that explains or solves the equation. And they duel with equations, looking for errors in others and their own, leading to new out-of-balance or unexplained observations, and the process continues. It's how the fields of inquiry work, a major part of the process. Einstein's cosmological constant is an unusually interesting case study in this.
Guess what? maybe the equation is flawed!
And maybe the flaw is that it is flawed.
Either way, we have do more than assert there is an error in an explanation or argument, else we're just nay-saying, rather than engaging in discussion and rational argument.