they are making an invenstment in a franchise, this marketing push is for this installment and the next ones. as well, Destiny doesnt have to cover up the entire marketing budget with the first installment. 

they are probably gonna release sequels to it instead of expansions. they are just commiting to the franchise for 10 years or so. 

the contract was leaked ages ago... every 2 years a new game (4 titles) and inbetween the years an expansion pack
http://destiny.wikia.com/wiki/Bungie-Activision_Contract

I don't think they will. I think they'll add planets with new story lines and missions.

Read post above you

Looks like I stand corrected. I find this a really odd strategy. If the engine is robust and it's designed with expansion in mind, why release a new game every two years when you already have the perfect set up to expand the series by adding planets?

No big risk.
If u spend enough on marketing and understand the market u can easily estimate sales.

Activision is betting on the franchise and not just one game. If all goes according to plan, they'll have another cash cow in their hands. Once you build up a franchise to the level COD, unless it's a fad type game like guitar hero, it'll easily make a few billion in its lifetime.

easy, new game = more hype = more sales = more money. 

also, instead of releasing an expansion at 30$, they release a new game at 60$. 

It's a long term investment:O