This would only be a nuisance, and nothing more. If you spawn in a cold/hot biome, you are now going to have to run out of it, or try your hardest to kill a herd or two of cows before you die, which will probably be unlikely. More often than not, people would just make a new world. If you don't spawn in one, you simply avoid it until you get armor, and then you never have to worry about it again. What's the point exactly? It's not "difficult" to go and punch cows for leather. It's not "difficult" to simply not go into a biome.
Plus, the reason you give in your latest comment makes no sense at all. You want to make the game more realistic because the game has been made unrealistic, changing what it is an has always been, basically, so people that aren't even gamers can appreciate the game more? What kind of sense does that make?
The game has a survival mode, yes, but the main draw of the game is the sandbox functionality. Some people like the little survival additions to make their stuff feel more earned instead of just spawning it in, but blocking off areas of the world is not going to help anyone like the game more.