There are two parts to the problem of a king size mattress developing a hump up the middle.
The part of the problem that is caused by the edges of the two boxsprings can, in fact, be solved with a platform bed.
The other part is that the two people sleeping in the bed, tend to sleep farther apart in a king.
The upholstery in the center remains like new, while the padding under your bodies take a body impression.
Most of today's mattresses, especially those made with memory foam, will always be subject to the later.
In order to minimize the problem, start with higher quality components.
All Natural Latex, for instance, instead of memory foam will help dramatically with the second cause.
My wife and I are large people. The king size bed we have is made by Natura of Canada.
Their Greenspring line uses a double tempered pocketed coil unit and is upholstered with natural Talalay latex.
We opted for the LIberty Ultra Plush model. We've had this mattress since last year...and, it has no body impressions and no hump up the middle.
The key is quality components.
Here is a link to that line:
Factory Direct to You....
All Items Made Factory Fresh to Fill Your Order
Free Delivery Nationwide....NO SALES TAX.