Based on what text in the Charter?
> When a country splits up, as the USSR did, the legal precedent is for all of the resulting states to come to an agreement on succession.
“Legal precedent” means something was challenged and ruled on by a court or similar adjudicative body prior to the time relevant to the breakup of the USSR; please cite the case? In any case, the objection is not based on “legal precedent”, but on the express text of the Charter.
(Also, the consensus-of-successors approach you describe is not the practical precedent at the UN; other split-ups that do not leave the original entity intact but with smaller territory have had the UN seat lapse and the new states separately admitted. Of course, none of those have held permanent UNSC seats; the breakup of the USSR was a sui generis case for which there really was no precedent, legal or practical, in that respect.)
The behavior of every nation in the UN over the last 30 years which has accepted them as such on the Security Council?
Only now do you have some idiots on social media trying to start WW3 by suggesting they be removed.