A simplified API means higher programmer productivity, higher robustness, simplified debugging and testing, and also less internal complexity in the driver. All this together may also result in slightly higher performance, but it's not the main goal. You might gain a couple hundred microseconds per frame as a side effect of the simpler code, but if your use case already perfectly fits the 'modern subset' of Vulkan or D3D12, the performance gains will be deep in 'diminishing returns area' and hardly noticeable in the frame rate. It's mostly about secondary effects by making the programmer's life easier on both sides of the API.

The cost/compromise is dropping support for outdated GPUs.