Is it really not worth improving a home because it is old and the improvement will eventually become old too and it is all pointless?

Don't buy toilet paper either.  You will just run out.

If your home is in a neighborhood that is declining, you can bet that what you put in it will not be recovered when you go to sell it.  However, there are things you must repair if you do sell.  

You sound like the author of Ecclesiastes. 
Maybe reading it would help you get out of this "all is vain" mood.