If suddenly, everything about the universe could be scientifically explained, would we still yearn for answers as to what it all means?
How is one thing. Why is another. No substitution.
Are we existential by nature?
We're pattern-discerning machines. If we can connect dots that are there, we do. If there aren't enough dots to make a picture, we we make some up. (Look at the constellations.)
Religion, for many, offers meaning to people's lives.
For those who can't or won't find their own reason and purpose, there is always "Something larger than myself" to belong to, whether it's the Russian Orthodox Church, or the Boy Scouts, or the Fatherland or the Party or the Masons. They are then given rules to live by, goals to strive for, discipline, order, direction, instruction, correction, atonement, absolution, redemption --- the whole ready-made pattern.
Science can provide many of those things - though never so comprehensively and only to the practicing scientist.
Religion welcomes laymen, and even infants before they've had a chance to choose - which relieves the parents of any need to negotiate with autonomous persons.
It can contribute something essential to the human condition.
Like anorexia contributes to the gastro-intestinal condition.
If science replaced religion, would we all somehow stop our storytelling?
You mean all of literature, cinema, television, music videos, barroom bragging, fish-stories, dating site bios, advertising and stand-up comedy?
No, I really don't think so.
Would myths and legends cease to be believed,
Who believes them now?
The religious pick and choose from their own canon, believing what appeals to them or works in their favour, disregarding what doesn't. Clerics preach that way, apologists write their screeds that way, and every few years another pope comes out with another bull. In fact, I don't believe the majority of faithful even know most of what's in their mythology, any more than most American know what's in their constitution.
Can science replace religion?
Neither is slottable.
They're omnipresent, coexitent aspects of human culture.
They are the products of human minds - the means by which we make our condition.