Vlad,
Why is ''God just is and that's that'' plausible?
I think plausibility itself is an unreliable measure here.
The trouble is that everything in the universe which we can observe is contingent in some way. But Russell is declaring the universe necessary rather than contingent since no contingent thing '' just is ''.
Now, don't misunderstand me, I'm not saying the universe might not just be I'm saying that whatever it is about the universe that ''just is'' doesn't seem to be being observed.
So whatever it is about the universe that ''just is'' is not observable and a ''God who just is'' also not observable. In other words whatever it is that is really ''just is'' and not contingent on anything is not observable.
So why is what is necessary or ''just is'' not observable? Because observability involves that which is observed being contingent in that it would be affected by the observation.
In conclusion then is that Russell is wrong to suggest the universe ''just is'' without an internal explanation. It's status as the necessary entity would be that explanation. However we observe the universe, so whatever it is which is necessary about the universe is unobserved and therefore we have a second reason to charge Russell with being unwarranted in his claim.
And so a universe which is in some way necessary has to have a necessary entity and thus we end up with the same number of entities involved in Russell's declaration as we do in a created universe, which brings us to another problem.
Russell's declaration is the opposite of the proposal that a creator must have a creator who must have a creator which seems to be one belief that atheists hold while subscribing also to Occam's razor.
So there are obvious problems in Russell's declaration, There are problems for the necessity of the universe when all we can observe is contingency and there are problems with an infinite chain of creators.
The only explanation that avoids these issues is one necessary creator.