This is all quite reasonable on its own merits. The problem is when you say "okay, let's extrapolate migration data from it". It makes sense to say "well, linguistically, these things seem to be related to these other things in such a way that the first things look like slightly modified versions of the second things, therefore the second things came chronologically first". Obviously one has to stop where one's evidence stops. But you can't take the lack of evidence as meaning that there is no evidence whatsoever. That "Branibor" is a plausible antecedent for "Brandenburg" certainly lends weight to the idea that Slavs lived in the Brandenburg area before the Saxons and that their name for the area was eventually transmitted to Saxon/Germans in their own way. It does not allow us to create any kind of chronology of settlement, or to assume that because the Slavic name is the oldest that we know of, that it must be the original. You, of course, seem to be aware of this.Relatively original, of course
This is a matter of how deep you're willing to dig. On a grander scale of things, of course, no ethnic group can claim autochtonity on any territory. The "ethnic group" itself is too vague a term. This is especially true for Indo-Europeans.
But on a smaller scale, for research and ego-boosting purposes, we can say that a territory held a certain period of time by certain people is "their". We can take first written accounts of toponyms as a base. What was before we don't know. Who knows what Leipzig was called before? But one thing we know - in German language this name doesn't make sense. It's a pronounciation of a perfectly understandible to any Slavic-speaking person name Lipetsk, meaning place of linden-trees. Or Brandenburg, which also doesn't translate to German iirc, but is a Slavic Branibor, meaning a pine grove of battle.
tl;dr: I guess we agree?
(satire on Polish government and Czech humor at the same time):
