If the Infinity Stones created the universal timeline, then what happens if they are destroyed?

We aren't sure at the moment, however we know that the timeline is ok(?¹).

My understanding was that what the Ancient One was talking about wasn't the flow of time itself but implying that the Infinity Stones create a barrier or protection against other forces and that if Banner took the Time Stone, this barrier would be broken leaving that point onward vulnerable to "The Darkness" and create a split in the timeline with a new sequence of events. this is what the black line splitting off the main one represented.

Banner however explained that they would return the Time Stone back to when they took it, maintaining the established timeline and not creating this split and this is why the black line vanished when he put the stone back. in regards to the timeline the stones were never really gone¹

in a sense i took the Ancient One's explanation as that the Infinity Stones act like something similar to how the Sanctuaries are a defence against The Dark Dimension.

but of cause considering that the Ancient One knew of Strange before he even had his accident she might already have known that he would use the Time Stone against Dormammu and stop the Hong Kong Sanctuary from being destroyed and the split she shows is that of Earth being taken into the Dark Dimension. however she was talking about all the stones, not just the Time Stone which makes me think she was talking about more than Dormammu and the Dark Dimension

_{1: this ignores the fact that Loki escaped with the Tesseract in New York after he was captured. we don't know if the timeline changed because of this or if Loki was recaptured}