In their case, preserving the original finish was not an option, since it was already pretty much gone. If the original finish had been in good enough condition to retain, I would have opted to retain it regardless of whether my intent was to preserve as many of the chairs' original qualities as possible or if these pieces were destined for everyday use. The rationale here is that original finishes, including original upholstery, add monetary and, sometimes, aesthetic value to furniture. Obviously, if the original upholstery is so badly worn as to be unusable, one may not consider it to hold any aesthetic value, even though it may well increase the monetary value of the piece to a collector. If your intention is to preserve as much monetary value in the piece as is possible, you may want to reconsider permitting the piece to be in regular use in your home, since preserving monetary value and allowing daily use can be mutually exclusive notions.
Regular use is bound to degrade the original finish.
However, if you are determined to use the piece, allowing the original finish to degrade through use is no worse than deliberately removing it by stripping the piece. A shellac or varnish finish that has survived for decades will certainly hold up under everyday use a bit longer. At the very least, you will delay having to strip and refinish the piece. In the case of these chairs, my objective was to recreate the original finish to the extent possible, but also allow them to be in daily use if desired. Enough of the original finish remained on the underside of the seat aprons to allow me to approximate the color.