Rainbows have always been circular, but usually appear as an arc because the bottom of the circle is blocked by the horizon. It is still possible to see a full-circle rainbow under the right conditions - typically from a high elevation when the sun is low in the sky. No matter the "true" shape they will always be just as beautiful and awe inspiring, but one must ask - if the rainbow is circular, where does one look for the pot of gold?
Rainbows are rainbows. It makes sense that they could be circular and there is no reason to believe that people will really think any differntly about them. This might change the legend about gold and leprecauns at the end of the rainbow, but most people won't care one way or another.
A rainbow will always be a rainbow whether someone claims that it is "actually circular" or not. We will continue to view it as we view it. It will always be a reminder of the covenant that God made with Noah when he said he would not destroy the world by water again.
No, we will always view rainbows as the beautiful oddities of nature they always have been. When other natural phenomena such as solar eclipses, meteor showers and aurora received their scientific explanation, it did not fundamentally change the basic human "awe factor". Making edits to the science behind rainbows will not detract from their appeal in any way.