Why can you not add herbs and spices powder at end?
That's absurd, imho.  You add spices generally to the earlier stages of a stew so they can meld in together, between themselves and with the main ingredients.  That takes time usually, more time than just cooking even.  It's why most stews are better the next day, when flavors have really had the opportunity to form a cohesive whole.

So if you add spices at the end, you run the risk of them not being totally well mixed.  But there's no way your curry would start to smell bad!  Especially from chili, which if anything would prevent any bad smell from its conservation powers. That's just superstition.  And I hope you added some and demonstrated that :-).  Or at least try it on your leftovers.