Some of them will, but not over night. Take MySpace as an example, it fizzled out after losing relevance. YouTube might die one day just like MySpace and nobody will notice, because nobody used it in ages at that point. Wikipedia is a community project, maybe one day someone there fucks up and some people will start a fork that becomes more popular than the original. I’m sure there are already some Wikipedia forks, but at the moment nobody cares about those.