Ease and convenience is so NOT what classic restorations are about. It would have been easier to do what you are saying, and even easier to just buy a new car off the lot, but it is not the same. It is not “taboo” or “unacceptable to the true spirit”. It’s simply not a restoration of a classic vehicle if you’re not actually restoring a classic vehicle.
What would have been a better choice is to select a vehicle that is in better restoration shape, but obviously as the years tick by, this becomes an increasingly large challenge.
If the engine and transmission are missing or not functional, obviously they need to be replaced. If the frame, body panels, etc are rusted out and have lost structural integrity, they need to be replaced. Electrical malfunctions need to be fixed. Upholstery needs to be replaced if in bad shape. All of the things you list need to be functional in a car for it to run properly and look like the original car.
Some people are so anal (for lack of a better word) that they actually hunt down original parts (scrapped from other cars of the same make, model, and year) to make replacements. Again, as time goes on this eventually becomes impossible.
Perhaps you are wondering why people bother. I can’t speak for others, but for me, I love old stuff. It’s pretty cool to see how technology has evolved over time, and looking at historical artifacts is the most interesting way for me to see this, probably because I am a very sensory person (looking at pictures in a book just is not the same for me). It is pretty amazing how much simpler cars were. Now, when you look under a hood, all kinds of stuff are packed in, but I’ve seen old cars that have just a handful of parts, where everything is mechanical, and there is more space under the hood than parts. Things were made better in the past. And even when the technology is truly better in modern times, it is still interesting to see the evolutionary process of the technology, to experience how people lived in times past. And so I think it is worthwhile to take the time to restore old things and keep them around.