While it is true that they're not destroyed by getting tossed to a random location on the Astral Plane, they shouldn't get XP for getting there. What they get XP for is (a) surviving encounters on the Astral Plane and possibly (b) getting back to where they were - could make an entire segment of a campaign about getting to somewhere where the party can get out of the Astral Plane, depending on their level.
The phylactery is not automatically destroyed (something could however happen to it on the Astral Plane that'd be bad for the lich) ... so he will return again and again, until you go to the Astral Plane and pop off Find the Path to the phylactery of <insert lich's true name>. Or ... you seal him in antimagic chamber and have everything that he might possibly use to escape or harm himself removed.Used to have a DM who abused the bag in a hole trick. It is assumed that everything in the bag is lost just like the bag, though there is no mention that the bag or any of its contents are destroyed.
So what is a Lich has his phylactery within the bag. It gets thrown who knows where and lost forever. Can the Lich be slain now? What happens if you do suceed in destroying the Lich's body.