Check your deed. You may own less than you think. My first house in TX had a sidewalk in front but it was actually on city owned property. My property line was about a foot behind the sidewalk, not at the curb like you might think.
You may have to pay for it as an assessment or in your local taxes. You will have to maintain it -shovel snow and/or edge it. You basically will have no say on who uses it and you will have to keep your sprinklers from spraying on it. Hard to fight city hall, students need safe walkways, residents need to stay out of the street, etc.
they wont build it without proper easements.. this is a win.. keeps you from having to pay for it.
When the city puts a side walk on your street, it adds value to your property either financially or visually.
The side walk will stay on the legal easement. If home owners have planted trees on the easement the city will remove them.