Actually, like all forms of slang, Cockney is continually evolving. Old terms fall out of use when their meaning becomes obscured by time like if an old term referred to a famous person who is no longer recognized in the modern age, then the rhyme becomes meaningless so something new has to be created to replace it. The changing culture of an area also changes the slang. How many white Cockneys actually still live in the areas that are traditionally associated with them these days? They're all from the West Indies and Muslim countries in those parts now so references from white British culture are meaningless to them. There's probably not much left from the 60's and 70's Cockney that's still in use today.
I see what you mean but at the same time i don't think the original terms become meaningless, its just less people know where they came from and the meaning behind them. For example, i've been Iron for Poof since i was a kid, never knew what it meant til quite recently and if i hadn't been of an inquisitive inclination i probably would never have. Same with loaf, use your loaf, i just never made the connection, never thought about it, it was just something commonly said. Slang evolves, yes but there's a difference between evolution and a bunch of new age re-ordering of an existing thingiemawotsit.
) the top one 'Yarmouth Bloater' = 'Motor.' 
Made me wonder if they would be doing that if others were around, or just to get me to freak out! 
The worst was when I went back to get an up-to-date maths qualification, a few years ago. Some of the things two young ladies in the classroom were reading aloud from Facebook... just made you think 'THAT can't be on the curriculum!!'