That large randomness is exactly what I like about banishment, and I wish more god abilities had it.
yesno wrote:cast it maybe 2 or 3 times and then stop casting since you don't want to lose corrupt, and accept that you have spent your piety for nothing.
If I use pacification or healing or upheaval or brothers in arms or greater servant 2 or 3 times and it fails to kill a dangerous monster and I have to run away, I also have to accept that I've "spent my piety for nothing" (unless I used it against a monster that doesn't regenerate, I suppose). If I use might or healing or any other resource and fail to kill the monster and have to run away, I have to accept that I've spent that resource "for nothing". If you want to present this as a problem, you'd better have a good case for it, because it would mean the entire consumable and piety systems are broken, and fixing it would involve
much more than removing the piety cost from failed banishment.
But more importantly: fundamentally, how hexes work in DCSS is that either the monster dies, or nothing happens but you still incur the cost. Still incurring the cost is a vital part of making the fail rate important. If the cost is only incurred when the hex succeeds, or the cost is insignificant, then to beat monsters with high MR, you don't need to increase your hex power to get a
good success rate, you just have to get a non-abysmal success rate like 10% and then spam the hex until it works. You can see this problem already with hex spells: to kill death yaks, enchanters just kite them around and use EH or Confuse until it works, even though the success rate is bad, because the MP cost isn't much of a cost. There are a few monsters that are too dangerous to try this on, such as spriggans, but there are far more monsters where it works perfectly well.
The same does not happen with wand hexes or banishment, because the cost when those fail isn't something you just shrug off like MP or hunger. This makes the fail rate much more meaningful and prevents the aforementioned degenerate gameplay.
You can say the hex system is a terrible design and needs to be overhauled, and perhaps you could even get me to agree with you. But you aren't saying that. You just want one of the hexes where the failure cost matters to be changed into one of the hexes where the failure cost doesn't matter. That will result in more of this tedious, degenerate gameplay, so I am confident that it is a bad idea. A further downside is that this change would make banishment inconsistent with how every other hex in the game works.
While you could keep the failure rate relevant by dividing the ultimate piety cost of banishment by the success rate, that would make it even more inconsistent with other hexes and even more difficult to communicate to the player. It also calls into question what should happen if you use banishment with a 1% success rate and it succeeds; should it instantly excommunicate you? Should banishment without enough piety to cover the cost be forbidden entirely? How do you communicate
that to the player? What about when a monster has unknown MR gear so the reported success rate is inaccurate? It's much simpler to let the failure rate matter by leaving the piety cost on failure.
It seems like your argument boils down to "spending piety on failed banishment feels bad as a player", but pretty much everything that hurts your character feels bad as a player. Dying feels bad, miscasting a spell at an important time feels bad, failing to kill a low-HP monster with your attack feels bad, yet these are all vitally important game mechanics.