If you mean that there are arguments against the Aristotelean concept of an unmoved mover, that is certainly so.
But typically the term "discredited" implies a broad public consensus, whereas many people accept some form of the cosmological argument, which is related to the concept of an unmoved mover.
To use your version of "discredited," we might say that both theism and atheism are discredited, merely because there are arguments against each.
Aristotle's unmoved mover argument (and it's slight variation in Aquinas' cosmological argument, and the whole class of arguments for the existence of God of similar general form) are discredited in the sense that they either (1) rely on equivocation, or (2) are internally inconsistent, or (3) simply introduce an additional unproven entity without explaining anything, or (4) even if one assumes it's completely true, it doesn't prove the existence of anything like the usual conception of a personal God anyhow. (The options aren't mutually exclusive, though usually onky one of the first two applies, depending on exact definitions of terms—though often those definitions are unclear enough to be uncertain which of the two applies to a particular argument.)
If "everything in existence depends on something else" is a valid generalization, introducing an entity with out such dependencies is a cobtradiction.
OTOH, if it's not a valid generalization, any number of things in existence could lack such a dependency, and there is no need to assume a unique and special unobserved entity with no dependencies.
An independent common first cause or unmoved mover is, perhaps, an aesthetically pleasing (for some) solution to the apparent problem that things generally seem to have causes but infinite regress is an unappealing belief. But it is neither logically necessary, nor does any first cause necessary resemble God (the Big Bang works fine as an uncaused cause on which all other events in the universe depends, and isn't anything like any traditional concept of God, as, more generally, does any initial state inferred from modern observations of physics.)
>If "everything in existence depends on something else" is a valid generalization, introducing an entity with out such dependencies is a cobtradiction.
Sure, but Aristotle, Aquinas and Leibniz did not this that this was a valid generalization.
>the Big Bang works fine as an uncaused cause on which all other events in the universe depends
It would not work fine as an uncaused cause, since it is not purely actual.
Also, it's worth noting that the unmoved mover argument doesn't actually make any reference to the universe as such. It starts with any given instance of motion and then argues that this motion must ultimately have originated from an unmoved mover. Neither the universe nor its beginning in time is ever mentioned. (And in fact Aristotle thought that the universe was past eternal, and Aquinas was neutral on that issue.)
1) You can be an atheist and still think that the cosmological argument is a good argument. In other words, you can think that the premises are plausibly true, and that the conclusion is significantly more likely to be true given the truth of the premises than it is otherwise. Being an atheist only prevents you from thinking that the cosmological argument is strong enough to outweigh all arguments against the existence of God.
2) Few philosophers are experts in the cosmological argument anyway, so it doesn't matter what most philosophers think of it.
If you mean that there are arguments against the Aristotelean concept of an unmoved mover, that is certainly so.
But typically the term "discredited" implies a broad public consensus, whereas many people accept some form of the cosmological argument, which is related to the concept of an unmoved mover.
To use your version of "discredited," we might say that both theism and atheism are discredited, merely because there are arguments against each.