The thing is having bad mobility was a legit weakness that some characters have and that's why it's balanced.
If you gave every character a sprint ability, mobility isn't even weakness and more of a slight downside.
Imagine if TF2 for some reason gave every character sprint, what's the point of Scout? Also Heavy would no longer have to worry about dying and taking too long to get back into the fight.
It causes less strategy and more chaos, but chaos shouldn't be that extreme in a team-based shooter where each character has a strength and weakness. It's more controlled and allows better plays.
