Since the announcement of the cancellation ppl are using their bitcoins to buy altcoins, that's why the price decreased (and not plummeted, it is still 7138$ right now). Look at the long run. I still believe bitcoin is the safest crypto to invest in though the cancellation of the fork will definitely benefit the altcoins in the short term
This is wrong, price of altcoins has no direct impact on Bitcoin's price. During this spring we had a huge altcoin rally which led to many of them increasing in their price more than 10 times and Bitcoin's marketshare dropped to historical low of 35%. By your logic Bitcoin's USD value should have crashed, but in fact it was the opposite - it was growing in $2,000 zone.
Bitcoin's price decreased mostly because the market operates on a "buy the rumors, sell the news" principle. Some traders knew that SegWit2x will be cancelled before it was announced, so they were the ones to likely sell to grab their profit before others can. And keep in mind that Bitcoin is so close to ATH, that any small drops can be viewed as natural volatility.