As someone who did contribute to NLTK quite a bit, it was quite useful back in the day especially when I had to teach NLP/CL to linguistics (non-CS) graduate students. I agree with Radim that NLTK has a purpose - and it's not to implement the latest and the greatest NLP algorithms. I'm glad NLTK exists and although it is not what I use today, I'm pretty sure whatever I do use today (CoreNLP, gensim, etc.) will all be superseded by the next best thing a decade from now.