Agreed, internals of the frame come first as any external corrosion is unlikely to go much further & certainly shouldn't become structural if the bike is taken care of in any reasonable manner. Simply put, I wouldn't expect it to get worse unless you were to dump it outside in a leaky shed or long grass for a few years. Stored indoors & not used as a winter salt hack it ought to be fine.
Cosmetically, my preferred method to preserve "patina" is to give the whole thing a good wash & rub over the rust spots with WD40 and fine steel wool or tin foil to knock off the really rough spots, then wipe over the whole frame with an oily rag (My usual choice is 20w/50 as that's what I have around but I've heard a lot of good things about boiled linseed oil)