Question
how feasible do you think it is to remove a 50 year old sycamore tree from the street side of our sidewalk? This tree is approximately 80ft tall and is 30ft from the front of our home built in 1948. We are thinking of a replacement.  

Answer
The first question I would have is-- Why do you want to remove it? Is it dying , causing problem with the roots and the driveway or sidewalk, etc. It took a long time to get this tree to this size and you will be losing a great deal of shade to protect your house in the summer.

It will be a large job that only should be done my a professional tree service that is bonded as to not fall the tree on the house. It will also be fairly expensive and if the tree is healthy I would not spend the money for tree removal and then spend again to plant another tree. The stump will need to be at least ground down and you will be able to plant the new tree in the same stop since the roots and stump will be in the way.  Over time the place where the stump is will decay and the spot will sink and will need to be filled with soil.

I would if the tree is healthy keep it and continue the health with an application of fertilizer once a year. Yes it is feasible BUT a very large job that I would not do if it were my tree.