Most things are included, but there are added extras such as tours, spa, specialty dining, internet and shops.
I am not sure I could willingly spend $1,000 on tours and I am not big on the spa and I find spending money on specialty dining at the luxury level offensive.
Possibly by giving the onboard credit Silversea is trying not to have price reductions while effectively having one. I dont' want to turn this into a blog post, but there are economic reasons why the onboard credit is better for the cruise line than a reduced price.
No dobut they prefer the onboard credit, to reducing the price.
Still depending on the length of the cruise I don't think it would be that difficult to spend the credit. Shore excursions alone could put a good bite into it, depending on itinerary. Add on the odd spa treatment for those inclined, and perhaps a more premium wine choice, and poof it's spent