The problems with this victory condition are:
a) Obviously no one but Vargagg could possible win at that point, and if Vargagg had lost then everyone would have lost, which isn't really that fun.
b) Usually it's not going to matter, since when the last player is the first player (and thus controls roaming), there is almost always a clear path to the entrance.
Of course, Dungeon Run is a game about player interaction, so I think house rules are cool if everyone playing is on board with it.
PrometheusLKR is Magos the Wise in Rallul's Banquet. Magos is never deceived.