Example:
I mine an Altcoin:
Mine 1000 ALT
Sell 1000 ALT at 500 Satoshis.
Someone just bought 1000 ALT, someone just sold 1000 ALT. BTC is unaffected. This is like saying "stock trading hurts USD".
Opportunity cost: you could buy bitcoins intead and give btc more value.
Each time someone leaves btc to invest in stock markets or something else, it hurts because the selling pressure. Stock trading also hurt btc with the opportunity cost issue, and btc hurts the stock market. So few to invest, and so many investiments.
Yes, but when someone sells their coins, someone has to buy the coins, or else there is no trade. For every Sell there is a Buy.
...and every sell drives the price down, which hurts Bitcoin: currencies are unusual in that their utility increases the more they're worth.
http://themisescircle.org/blog/2014/02/12/im-hoarding-bitcoins-and-no-you-cant-have-any/
...and every buy drives the price up, which helps Bitcoin
When someone buys an altcoin, someone is also buying Bitcoin!