There can't be one size fits all in Linux. The very essence of Linux is have multiple distros and people creating offshoots. That may be the sole reason in the end that Linux may not completely replace Windows! However, that is also the strength of Linux.
One good example of why one size fits all is a bad idea, is that in today's world most people will discard the old P3 with 128 MB RAM. However, Puppy make it crisply working system, and in style too.
Some students who come from a village to study at my Univ in Delhi, bought an old P3 with a 2 GB (yes, no typo) hard disk (running windows 98), for a paltry Rs 2000 ($ 40). They couldn't even use their pendrives on it, and could not install any useful software in that little space, whereas their friends made fun of them. After I handed them a remastered Puppy CD, they are thrilled to bits. They use internet, watch movies off pendrives, and listen to mp3 music. The friends who were laughing at them, stare at Abiword springing up immediately on the icon click. And believe me, in our country there are many who can only afford such a system. Which size will fit this category, except Puppy (or DSL)?