So sorry op. I'm not a doctor or hcp but did some training for hospice volunteering, and what you describe is a quite natural death for someone of your dear gran's age. It sounds like the hcps didn't properly explain to you how death was going to progress, this can make it incredibly confusing and upsetting, I'm so sorry.
A drip might have kept her alive for a few extra days or weeks, but the hcps will have been aware that those extra days will have been less comfortable, even more painful and distressing, for your gran and they wanted to save her from that.
It's a very hard one, in our culture we tend to get muddled about death and don't always remember that quality of life/comfort at the end of life, might be more important than getting those few extra days.
I know you feel guilty - i'm so sorry - my gentle suggestion is that maybe your guilt is a worthy "price" you're paying in return for sparing your gran unnecessary suffering before she had to leave this world. In time you'll see that even that "price" isn't one you have to pay, if that makes sense - you've nothing to feel guilty about, not at all. You actually did a kind thing for your gran by trusting the hcps, I think.
I know it doesn't feel like that right now. I'm really sorry. xx