The signals are all pointing to the SegWit activation being ALMOST certain! Many speculate there will be a huge spike in BTC price on August 1st if this works out. Some think that some altcoins will drop in price due to this, which may occur, but I believe that the most powerful altcoins won't be hit too hard and, if they are, I believe it will be because selloffs are occurring so that people can step into bitcoin during the spike and they'll correct back shortly after.
That being said, many trading strategists believe that it is a dangerous game to try and speculate on the way news will affect the market, as counterintuitive situations can occur very easily in these scenarios. An example is a whale who holds a great deal of BTC; they could cause drops through huge sell orders despite the news, making people panic and leave, so they can grab at a cheaper price before the spike that would have occurred otherwise. If you had weak hands during the drop, you could lose big. Additionally, other factors can always come into play and cause drops in prices. My personal opinion is to hold some BTC into Aug 1st, but I am not advising you to do the same, as I'm not a financial advisor.