I would disagree. I think at least some cards should be hard/impossible to get as free players. Other games make it too easy to get by as a free player with little to no disadvantage. Maybe a lot of people like that, but I'd prefer the game gives more reason for myself and others to support it. I just wish they would do it in ways that are not completely RNG. If there is anyone on this planet that simply does NOT belong in a casino, that would be me.
Just give the game some time, and even free players will start getting high SED levels, just that it will be at a much slower pace. As cards get added over time, these boss events keep getting easier as our total SED keeps rising. Eventually it will be at a point where even without slayer cards, 8.7 mil bosses will be a breeze. Even so, from what I can gather, there is always some kind of slayer card(s) available to free players.