Historically, how do we know he died at all? Spartacus, though his 6000 man army was died in their final battle for freedom and survivors crucified, they never found Spartacus body, though there was no lack of people who would be able to identify him, which left a sour taste in Marcus Crasus mouth. Josephus records the fate of Jonatan Gush Khalav, imprisonment and life in the salt mine and Shimon Bar Giora was dragged by a halter around his neck to the Mamertine underground prison (second level added later) and strangled there.
Since it was common Roman practice to at least confirm the death of an enemy, the fact that no history's records such a identity of any of the dead as being the Prince of Israel, means that among other possibilities, the body too mutilated to be sure of who it was. But knowing that part of the revolts strategy was building tunnels to escape right under the Romans feet to safe distances and the fact that Cassius Dio does not even mention Bar Kohba or Beitar though he describes the whole campaign including bringing Julius Severus from Britain.
13 1 At first the Romans took no account of them. Soon, however, all Judaea had been stirred up, and the Jews everywhere were showing signs of disturbance, were gathering together, and giving evidence of great hostility to the Romans, partly by secret and partly by overt acts; 2 many outside nations, too, were joining them through eagerness for gain, and the whole earth, one might almost say, was being stirred up over the matter. Then, indeed, Hadrian sent against them his best generals. First of these was Julius Severus, who was dispatched from Britain, where he was governor, against the Jews. 3 Severus did not venture to attack his opponents in the open at any one point, in view of their numbers and their desperation, but by intercepting small groups, thanks to the number of his soldiers and his under-officers, and by depriving them of food and shutting them up, he was able, rather slowly, to be sure, but with comparatively little danger, to crush, exhaust and exterminate them. Very few of them in fact survived. 14 1 Fifty of their most important outposts and nine hundred and eighty-five of their most famous villages were p451razed to the ground. Five hundred and eighty thousand men were slain in the various raids and battles, and the number of those that perished by famine, disease and fire was past finding out. 2 Thus nearly the whole of Judaea was made desolate, a result of which the people had had forewarning before the war. For the tomb of Solomon, which the Jews regard as an object of veneration, fell to pieces of itself and collapsed, and many wolves and hyenas rushed howling into their cities. 3 Many Romans, moreover, perished in this war. Therefore Hadrian in writing to the senate did not employ the opening phrase commonly affected by the emperors, "If you and our children are in health, it is well; I and the legions are in health."
4 He sent Severus into Bithynia, which needed no armed force but a governor and leader who was just and prudent and a man of rank. All this qualifications Severus possessed. And he managed and administer both their private and their public affairs in such a manner that we are still, even to‑day, wont to remember him. Pamphylia, in place Bithynia, was given to the senate and made assignable by lot.
15 1 This, then, was the end of the war with the Jews. A second war was begun by the Alani (they are Massagetae) at the instigation of Pharasmanes. It p453caused dire injury to the Albanian territory and Media, and then involved Armenia and Cappadocia; after which, as the Alani were not only persuaded by gifts from Vologaesus but also stood in dread of Flavius Arrianus, the governor of Cappadocia, it came to a stop.
In fact Marcus Aurelius Tutor in a letter to his pupil gave a more somber opinion of Romes defeat of the Jews.
"The god who begat the great Roman race has no compunction in suffering us to faint at times and be defeated and wounded. [...] But always and everywhere he turned our sorrows into successes and our terrors into triumphs. But not to hark back too far into ancient times, I will take instances from your own family. [...] Under the rule of your grandfather Hadrian, what a significant number of soldiers were killed by the Jews.
[translation C.R. Haines]
The interesting aspect about this letter, that was written in 162, is that it presents the war against Bar Kochba as a military defeat."
So did he die in battle or did he manage to get thru the tunnel system or or means past the Romans who were probably not up to snuff after so many battles and left perimeter security lax.
Indeed, maybe Bar Kocba survived after all.