The £15m generated is to pay off the stadium debt http://www.arsenal.com/news/news-archive/wenger-we-are-paying-back-our-stadium-debt
From an investment point of view, it is a high risk, when considered against putting the money in a savings account, or a company in a more liquid market. If you don't know why, or refuse to see why, there isn't much point in me trying to explain. Just remember, no one is saying that he will lose ALL his money. Just that the chances of him getting full value, if he wants to sell, is very low.
All this talk of having to have £15m profit is based on a misunderstanding of what Wenger said. He was taking about having to use £15m of the revenues to pay off stadium debt each year, as Piskie pointed out. It actually had nothing to do with profit.
I can see it now. Kroenke walks into the Halifax BSOC 'I've got £500m to invest - which 0.5% account would you recommend?'