I don't know how the RNG in this game is programmed, but I know there's an "average" amount of fails per success. This "average" does not apply to mob drops however, which is why I suggest that mob drops be governed by the rules of probability over time.

For instance, if an item has a 0.005% chance of dropping, every time you kill the mob that drops it, your probability is 0.005%. However, the laws of probability dictate that over time, your probability will reach 100% given enough attempts.

Therefore, attempting an item with a 0.005% of dropping would have a maximum probability of dropping in this fashion:
0.049% after 10 mobs killed of the type that drops the item.
0.49% after 100 mobs
4.87% after 1000 mobs
39.3% after 10000 mobs
99.326% after 100000 mobs

Thus, a super rare item could be acquired after killing 100000 mobs of the type that drops the item. Since most areas only have packs that include 1-3 of the mobs that drop an item of this rarity, you could say a total of 300-400,000 mobs total for a rare item drop. I believe this would balance the game more against those who are "carried" by RNG and those who need to grind their days away every day to get anywhere in this game. I know there are a lot of people who burn out knowing that someone walked into their rotation and got that rare potion piece after three packs of mobs while they're sitting on over 1 million mobs in that same area. I'm looking at you Ronaros Forest.

Essentially this would more or less "guarantee" (there will still be outliers) that if you put in the effort, you will get that rare drop you want. Right now, there's a large probability that some items will be unobtainable by some people in the game, which I think is grossly unfair to them who play the game the same way as everyone else does.

If you would like a link to a more visual calculation of this concept here is the link: Calculations
Change the number of attempts and the percentages as you want.

Not going to happen in any P2W Korean MMO. You might as well throw any thoughts on this subject, out the window. The RNG carried in these games. Are to serve as trucks. The rare item drops in these games. Are to serve as the bone. Your wallet is to serve as the dog chasing the bone, being pulled by the truck. EZ money, they don't even have have to produce content of sustenance.

By the laws of probability you're talking about, you need exactly infinite amount of atempts to reach 100%, hence in reality you never reach that.

In certain games there is luck portection features, which increases your drop rate over time, to prevent being unlucky, but matemathically you would never reach 100%.

The irony of someone that doesn't understand the laws of probability demanding that something follow the laws of probability. 

Not infinite. It's not calculus, the limit does not approach one, it reaches and exceeds 1 after enough attempts. Find the formula and do the math yourself. I put it at 150k attempts and it reached 100%. I imagine something lower than that would also reach 100% as well.

He's not demanding anything, just stating a fact. As someone who studied math for half her college career, he actually does bring up a valid argument. Probability over time is a very well-known and practiced concept. The game intentionally forces the RNG to not follow this concept and instead adopts a flat percent drop rate regardless of attempts. Meaning, a lot of people will never be able to get items that are in the game for everyone to get. 

It literally is calculus, the onyl way to get a guaranteed outcome is infinite attempts. The chance of something happening will never, in finite attempts, be guaranteed, only EXCEEDINGLY likely. The calculator rounds up because its 99.999999999999999999999% at that point. Still not guaranteed.

All right enstein, please tell me what positive number should I put into the x in the 1-0.99995^x equlation to make it "exceeds" 1?

Then you should never have gotten a degree, and your professors should have been fired, because you clearly did not learn the basics.  In fact they should fire all your math teachers down to at least 7th grade.