The cereal was around for a long time, and like other products, when it goes away, people will miss it. I think that general mills will be able to make alot of money from selling this, as die hard fans of the cereal will be dying to get their hands on it.
Everyone wanted it back and finally rather than create something new, why not bring back what people wanted and missed? People like what is familiar and also these days, cereals are an affordable food that is eaten throughout the day, not only for breakfast. French toast is popular, so it's a quick fix if you don't have time to make the real thing.
Yes it was a good idea for General Mills to bring back French Toast Crunch. It seems from their customers that it was in demand and will sell well upon a re-release. It also gives General Mills some good publicity to show that they care about their customers needs and desires.
Now I will admit fully that I, personally, was not a fan of French Toast Crunch cereal, and I think the reason General Mills decided to discontinue the product in the first place had to do with the fact that consumers didn't react very strongly to it. That said, it has been enough time for GM to cash in on the nostalgia factor, and it may end up paying dividends, even if the formula hasn't changed.