It's a little more nuanced than that. You're only thinking about the bear market cycle. The dynamic you're talking about actually pushes Bitcoin's price up during bull markets. During the 2017 bull market, people were buying BTC and ETH left and right so they could invest in ICO tokens. Altcoin speculation definitely increased demand for BTC and helped push its price higher than it otherwise would have gone.
Where would Bitcoin's price be if not for altcoin investors scooping BTC supply off the market? I'm guessing it would be lower than now.
I somewhat agree. Sadly, it was a short term phenomenon which did more harm than good regarding the big picture.
it looks like that now since we're so deep into the bear market, but money has been flowing back and forth between bitcoin and altcoins for years now, ever since litecoin started trading. 2017 just happened to be the biggest altcoin cycle yet. there's no doubt in my mind there will be another altcoin mania within another bitcoin bubble.
the only harm done is to the speculators looking to get rich quick. if they can't hold through a bear market (or trade their way through it), that's their problem.