If your item is a true antique of high value and in good condition, "a museum quality piece" as we would say, refinishing it would likely decrease it's value. However, 99% of what we see does not fit that category.
Most of what we see are not "true antiques" but simply old furniture. Most of it is post-industrial, twentieth
century, factory produced and not hand made, hand crafted items. Most of what we see has nospecial value in that it was neither designed nor made nor used by someone of historic significance. We recognize that it might be precious to you though, because it is your family's heirloom.
However, even an old garage sale item can have value in that it is undoubtedly much better made and more durable than today's
particle board and paper-laminate disposable furniture. Old furniture, properly protected and cared for should last at least another lifetime. But, realistically it has no value other than it's usefulness,
comparable replacement cost or sentimental value.
Most pertinent though, the furniture we see is usually not in "good condition." It ranges from fair, to endangered, to
unusable, to a basket case. Old finishes DO DIE. Not only can they become black and opaque, hiding the beauty and character of the item. They can fade, blister, peel, prune and otherwise deteriorate to the point
where they no longer serve their PRIMARY function of protecting the wood. Moisture starts splitting the grain. Old glue dries out and gives up. Veneer starts lifting or splitting, joints loosen and break.
Lumber warps and splits. All these problems can be dealt with effectively if addressed soon enough. However, as much as we hate to write off a piece, we have seen furniture neglected so long, that it is simply