"If you think value Mongolia at zero then TPD shareprice will not outperform STX so why not swap out now and avoid any black swan events" - capital gains tax would be one very valid reason I can think off. If the intention is to hold STX once converted from TPD, then better to hold your TPD now, get the roll over relief and not have to pay out of pocked CGT (if in profit) and also not reset the 12 month CGT discount clock.
regardless, you get the same amount of STX share at the end, regardless of which out performs which .