That means it's not wise to defend the historicity of Jesus by defending a particulartheory of historicity. It makes far more sense to class together several (or hopefully all or nearly all) theories of historicity by claims they make in common. For example, if all ten contradictory theories of historicity all make the same claim (e.g. that a man named Jesus who was later called Christ was crucified under Pontius Pilate), then the prior probability of 'historicity' per se would not be 1 in 10 but nearly 100%—if, that is, the prior probability of mythicism, and other theories of historicity, were nearly zero. Because the full 100% (the full probability-space) must be apportioned among all logically possible theories. But that means even if the prior probability of myth were as high as, say, 25%, we would still have in this case a prior probability of historicity of 75% (because 100% — 25% = 75%), as long as we stick with that minimum historical claim about Jesus. Otherwise, for each of the ten competing theories of historicity, unless we can demonstrate otherwise, the prior probability of any one of them being true is at best a paltry 7.5% (1/10th of the full 75%), far less than the 25% prior probability for myth (though again, only in this hypothetical scenario). If we classed 3 of the 10 theories together (by, let's say, the claim that Jesus was an apocalyptic prophet) and insisted on defending only that, then (all else being equal) we'd still have a prior probability of only maybe 25% (1/3rd of the whole 75%), which leaves us with equal odds against myth. So it may be better to drop unnecessary claims like that (no matter how well evidenced they may be) and focus only on what all or most theories of historicity assert in common. Because that will stand the best chance of showing the inadequacy of mythicism (if such can be done).
Carrier is conveniently assuming that the mythicist theory is monolithic. This is not the case. There are plenty of mythicist theories around, some of them rather old, others fairly recent (due to Doherty's revival of "mythicism").
Some (as Doherty & Dr. Carrier) have a Christ crucified in the sky, others do not (however Doherty explained Christianity originated from a layered Q, but Carrier has it very differently: see here
). Some acknowledge Paul existed and wrote authentic epistles (at the time indicated by 'Acts'), others do not (such as Dr. Robert M. Price & Dr. Thomas L. Brodie, with many, likely most of today mythicists). Some deny Christianity started in the 1st century, others do not. Some make Marcion the creator of Paul and Christianity, others do not.
Depending on the particular mythicist writer, a fictional Jesus has been modeled on Osiris, emperor Titus Julius Caesar, king Agrippa I, Judas the Galilean, Alexander Jannaeus, a pagan Christ, a composite figure, Elijah-Elisha stories, the sun-god, a dying & rising god, Pandera (a Roman soldier), a man of an indefinite past, etc.
Actually, the competing mythicist theories are many and show more differentiations between each other than the historicist ones. Even if the most popular of Jesus' myth theory was about a believed crucifixion in heaven, with Paul, Peter, John & James having existed, Jesus' historicity (as Jesus being an apocalyptic preacher) would come overwhelmingly on top. Therefore the "equal odds" of Carrier is very misleading and is just another one of his fallacies.