This is one of those linguistic pet peeves of older people about how younger people use the language. In Japanese, you’re supposed to say “sugoku oishii” (very delicious), but in the past several years, it’s become commonplace for people — particularly younger people — to change “sugoku” to “sugoi”. It may have started simply because it’s easier to say, or it may have started because teenagers like to be different. Regardless, I suppose it is fast becoming acceptable. I believe I even heard it used by a pundit on a news commentary panel the other day.
Here’s one example of how something like this happened over time in English. The example isn’t a mirror of the “sugoku” to “sugoi” change, but it is an example of how language evolves.
So what do you think about language evolution? Can you think of some other examples? Do you think “sugoi oishii” should be acceptable, even in the media and in textbooks/on tests?