With regards to Diablo there are no lootboxes in the game, and all of the offers for crests and materials you can buy state clearly what you get if you purchase any of them. With the exact amount of what you get.
So anyone trying to buy their way to more legendary gems in the hopes of getting a 5-star gem are still going to have the dice roll their way at the end of each elder rift. Whether you have 1 legendary crests from the pauper pass or spend thousands of dollars to buy a thousand crests. You still need to deal with the rng on the loot table. Thats why youre seeing streamers spending a lot of money and claim they still didnt get a 5-star gem.
The only way to safely buy the gem you want is if it is on the market house, because you can buy eternal orbs for real money, and trade those over to platinum which is the market currency. That may cost a lot, to buy the orbs and trade to platinum. But who does that? Things may change here though, so Im looking out for the same crap that happened with the d3 auction house and having to buy gear and weapons to progress beyond the lower to mid difficulties. But thats not the case though, there are only gems on sale in the market place. And you can craft them with a bit of patience. And luck, of course. I dont expect to get everything I want, in fact I was still farming for some legendary gems years into diablo 3 so thats part of the game design imo (also for World of Warcraft where I have few items that never dropped after hundreds of runs through the same raid or dungeon). Thats just how it goes sometimes, and Im not gonna start paying for any gems or gear. Thats ridiculous.
So the only way to regulate the mtx in Diablo Immortal would be to put a ban on bosses dropping RNG loot in games. Yes, an actual regulated ban against monsters in videogames dropping randomised loot. Thats what you have to do to get at Diablos ecoonmy, because elsewhere they are not breaking any rules with regards to not knowing what you pay for.
There is no gacha mechanic in the game, and there are no lootboxes you can buy. Everything that can be purchased it is clearly stated what it is.