That's not what it says though. It says a Soviet 4-5*, with a 40% chance of a 5*... a group which includes T-62, T-34/100, T-44, IS-3, IS-7 etc. as well as the event units. So it's more like a 4% of a specific tank assuming they have equal drop chances. Point is, it's not expressed clearly.
They could make it so you take no damage on an event stage clear. They could have it set up so your tanks take no damage on the first few attempts of each stage. They could do level scaling based on something rational like average unit level, or treat every unit used in the event as though it's base stats were at your Lieutenant's level. There are a lot of good alternatives between "unlimited attempts" and "go fuck yourself, new guy".
Thereby placing the highest whaling requirement on the least happy, least heavily invested portion of the player population. Even without that frustration people are going to need to whale if they want Kat or Eliza. I understand they need people to buy shit but again, there's better ways. Also just throwing this out there, but the game could add some cosmetic items. Ammo for colored smoke trails or fireworks artillery or something.
That's true. It's good for the players, and it also benefits them since you can bridge any of those stage gaps by spending cores.