There is enough evidence and facts that show that anyone who brings the mindset of wanting to make stable money from gambling always ends up with a downfall and many other problems. It is just a push of nonsense, and it makes sense as your suggestion if there are people who say like that then we will be able to believe if they provide real evidence, so we must be more careful.
Looking at the Op's perspective, gamblers shouldn't think a player can't make a living through gambling. Because as gamblers we are supposed to have met players who actually earn significant amounts through gambling. Don't know for online gamblers, but physical casinos comprises of these kinds of gamblers. And they also have comps that help gamblers to stay winning by earning points. On the other hand, it's not wrong to think of gamblers not winning consistently in gambling, especially for gamblers who haven't seen or read about such people. Somehow, these people who make a career out of gambling begin early to master how the machines work. Though risky for any other gambler, but a well strategized player that understand the house edge can make a consistent money via gambling. Your second paragraph is right, and it's one of the strategies of professional gamblers. They don't worry about winning big in gambling. The motive is enjoying the process and mastering slot machines or poker games. I grasp Op's obsession about such gamblers, as it sounds like they don't trust the power of gambling. The risk is high, but I keep telling people, that doesn't make it impossible.