I think it depends, if a loose structure is imposed on the party then people could arrive and leave rather easily.
Say that party journeys to place A with 6 people, along the way two go inactive, all that would be needed was a logical explanation and you could have them depart. For example Character Tom learned that his father was dying and had to go home.
Worst case characters could simply be killed off, i.e. party goes to fight some bandits, player goes inactive, character is hit by arrow falls into river and is assumed dead.
If structure is far too loose then I could see the RP dieing for exactly the same reason. As characters might not be anywhere near each other and no real "goal" or "story" would necessarily develop.