It's not an assumption, I understand the risks. I know it will go both up and down. That doesn't make it gambling. What on earth isn't gambling in your eyes?
understanding of risks doesn't cut the mustard... I understand the house always wins. Understanding the odds in blackjack and understanding the odds in the stock market still makes you a gambler...an informed gambler, but nonetheless a gambler.
So if 401ks are bad because they incentivize gambling, what's the alternative? You have a uselessly broad definition that seems to lead people only to a guarantee of losing purchasing power through inflation. Better guarantee a loss than "gamble" with strong evidence to support your risk I see.
Saying "the house always wins" about things like index investing is hilarious.