If we go from a generic concept of monopoly as consolidation of power instead of a specific instance of monopoly - this company has consolidated power - then it follows that if consolidation of power is more efficient we should just consolidate power.
But sure, if it's specific instances of consolidation that are more efficient then you are correct, the specific company has shown it can do it efficiently and thus it does not follow that consolidation with the government would be efficient.
but if you have a situation that what determines if a monopoly is efficient is that it is a monopoly and controls its industry then it seems we have just in a round about way gotten back to any consolidation of power is efficient and the government should do it. Unless you can show inefficient consolidation of power that is inefficient in some way other than by losing the power then it seems the argument for nationalization is back on.
on edit: on the other hand if you can show an inefficient consolidation of power, like consolidation of majority of shopping through one portal that is yet unable to keep counterfeit products out, then it argues that inefficient consolidation of power should be broken up because it's inefficiencies will not be solved as long as the power is consolidated and competition will hopefully help.