Holding BTC for the long term has historically been a lucrative investment. Just not holding it on one of the many centralized exchanges that always seem to get hacked or go bust.
Sure, the price will always go up in the long run. But it will become less and less liquid, and at some point those who are left will have hundreds of thousands or millions of theoretical money that's completely unspendable, because no-one else will buy in for more than they did.
I'm not a crypto expert but that statement seems incorrect to me, since as far as I know BTC has deflation built into it. The more is mined the harder it is to mine more. So as long as investors are keeping some trust in it and are not selling at large scale, the BTC value should continue to go up in the long run (ignoring smaller fluctuations). That's why people have been flocking to it in the first place.