How replayable is Pandemic Legacy? (Is the game still playable after you have completed it)
It is not intended to be replayable. 

Depending on what options you choose during the campaign, certain components are intended to be physically destroyed. You could choose to just set things aside instead of destroying them; though that does take away some of the thematic fun. And then you would just have to ignore any stickers that were put on the board during play if you wanted to play regular Pandemic after. 

You can either throw it out or keep it as a souvenir.  As a matter of pure opinion; it is well worth it even though you can’t keep playing it after; those 12-24 games are worth the cost!