Surely being a world cup stadium is not that big a deal, and certainly not reason alone to expand your stadium.
You may make a couple of mill and get a little bit of presige, but noone would spend tens or hundreds of millions just for that.
However, the reason Arsenal spent an absolute fortune moving to their new stadium and effectively stopped themselves winning trophies for a number of years, is that for a Champions League club to be self sufficient they need a big capacity and loads of corporate customers.
That's why we are looking at it, I assume.