Have you considered the PRG Amex? It doesn't offer the lounge access and other "extras" but it complements the Sapphire pretty well as it transfers to different partners and earns bonus points on different categories (2X on Gas and Groceries / 3X on airfare)
Personally, I think you could carry the PRG Amex, the SPG Amex, and the Chase Sapphire Preferred for less than the cost of the Platinum - and still earn points for use in the same programs. In fact, you would earn more MR points with the PRG than you would with the platinum.
That's pretty much what I do - Chase Sapphire for dining. PRG Amex for Gas / Groceries. SPG Amex for any "other" spending.