Well, if Sigismund had more support in Sweden, it might have worked...but the problem with this union was that the realms in question was religiously different. Sweden was Protestant, Poland-Lithuania was Catholic (and if the Time of Troubles had happened still, then you'd throw in Orthodox Russia into the mix). Now I dunno if Sigismund III was a diehard Catholic before or after the breakup of the union, but it is largely because Sigismund was Catholic that the union collapsed in of itself (for the same reason that the Glorious Revolution got rid of James II/VII of England/Scotland and eventually barred any Catholic from being put on the British throne afterwards).