I have to say this because opinions like these are always used to justify the status quo and are very detrimental to improving the game.
I feel the need to say that I am not apposed to improving the game. My first post even outlined changes that I would like to see made to the Great Person system. Yes, I am defending an existent system, but I'm doing so because its an existent system, I'm doing so because I think the aspects of that system I'm defending are fine. It's just that I like a certain system and you don't like that system, so the "improvements" you'd make and the "improvements" I'd make are going to be different and we'd prefer different things left be or changed.
However, I also don't mean to put words in your mouth, so forgive me if I have misconstrued your meaning.
Now, I understand if people don't like the Great People system and think its too out of the players control. I understand the desire for, in a game, things to be consistent reliable, and for a specific strategy to always work for you. However, I feel that in Civilization games that having wrenches in your plans throughout the game keeps things more interesting. I also understand if people think that passing on Great People versus choosing which way to explore are different, because they are.
Nonetheless, I wouldn't call Great People in Civ6 a "luck based system." It's a shot in the dark, I'll give you that, and in that regard there is an element of chance to it, but the player is also able to assess the risks in these "shots in the dark" and then choose which shot to take. You can look at a Great Person and determine how valuable their effects are to you before you make that choice. If it's an affect you want, you can take it. If it's one you don't, you can skip it. If it's a more middling effect, you might end up with something better or worse, but there is, again, weighing your decision to see if you want a benefit you know or one which could fall to either end of the quality spectrum. I personally find that an interesting dilemma for the player to have.
I think there are other aspects of the game like that which you can find throughout, and not just in Civ6. Late game strategic resources aren't something you can see right away, you don't know where they're going to be until you discover the appropriate technology. Some players will go ahead and found cities in deserts because they expect oil will be there, they weigh the likelihood of that versus the investment in the settler and building up that city and decide that the chance is good enough to go ahead and settle there. Other players won't settle a desert unless they do know such resources are there and wait until they have the appropriate technology and know it isn't their borders before they settle their desert city.
In both of these cases, a player is able to assess whether they want to perform a more risky action which could end up working out better and another where they know what they're getting. They aren't perfect analogs, certainly. Still, I think the player can accumulate enough knowledge in both circumstances (knowing what Great People are for each era compared to knowing the spawning pattern of strategic resources) to make these into interesting decisions.
But hey, that's just me.