I would keep the old bike only if it could easily be made comfortable by swapping a few components -- e.g., a new stem to raise handlebar height to about the same as the seat. Otherwise, there is no point to keeping a bike that doesn't fit or does not match one's riding style.

I have a Miyata 1000 that is as old as yours, but no longer use it for multi-day tours. (Now I use a custom built True North.) The Miyata is not as comfortable as my True North, and I find I much prefer brifters to down tube shifters.