It is indeed true, that unknown masterpieces can be lost because they are only kept on an undeveloped roll of film, but not likely because it is impossible to develop the film. Exposures on undeveloped films tend to fade much faster than development processes disappearing, so the image is likely gone even if the process is still available. If you happen to find a 40 years old exposed C41-film, I would not expect to find any usable images on it, even if it is developed in the still more or less readily available C41-process.
Even if an outdated process is not commercially available anymore, you are very likely to find processing instructions somewhere. It has never been common for producers of photographic material to keep the processes a secret and the Kodachrome K-14 process is for example described in detail in US patent 3,658,525. Even if the process is known, it is of course not practicable to do K-14 development at home in the bathroom. You will not only need a seemingly endless list of odd chemicals (the synthesis of the more stranger substances is described in the patent), but you will also need a specialized apparatus for the additional exposure. So after you have obtained sodium hexametaphosphate, sodium bromide, 5-nitrobenzimidazole nitrate, potassium iodide, sodium sulfite, sodium sulfate, sodium hydroxide, sodium thiocyanate, hydroxylamine sulfate, 4-amino-N-ethyl-N-β-hydroxyethyl-3-methyl aniline sulfate, 1-hydroxy-N-(2-acetamidophenetyl-)2-naphtamide, hexylene glycol, polyoxyethylene, N-benzyl-p-aminophenol, methanol, p-aminophenol and some distilled water just to make the cyan developer agent (that is one of seven baths), before you develop for 4:30 minutes at 27°C, you will have to flash the film with red light from the back, while making sure, that the front of the film is kept dark.