The deaths of both men has seen a huge number of tributes from other actors and writers, from people in the public eye and people on Facebook or Twitter who want to share publicly how important Nimoy and Pratchett were, not to mention how much their work entertained them and had an impact on their lives. It's funny - I'm not a huge SF fan and I've not read every single Discworld book, but I am one of those people who loves great characters and the relationships they have with others. I love the way Spock bounces off Kirk, Bones and all the other Trek characters; I love how you can sit down with those guys and watch them develop over decades. Ditto with the Discworld series. Sam Vimes, Rincewind, Polly and the other members of the Monstrous Regiment, Albert, Granny Weatherwax, Death (obviously) and God knows how many others - they're all people we share our lives with in the same way we share our lives with the crew of the Enterprise even if we don't give a monkey's about SF. Certain pieces of art find their way into our hearts; certain people stay with us even if we don't realise they were setting up home in our houses in the first place.
Leonard Nimoy became Spock, and he always struck me as a man who did so gladly. Terry Pratchett became his own world. The worlds of Trek and Discworld are silly and fun and frightening and serious and angry and worlds where you trust things will work out even if they take a while.
As long as we keep those worlds with us, they don't go away.