Getting rid of cruft isn't really a goal in and of itself, it's a goal in service of other goals. If it's not about performance, what else would be accomplished?
Getting rid of cruft and simplifying the GPU access, makes it easier to develope software that uses GPU's, like AI's, games ..etc.
Have you taken a look at the codebase of some game-engines, its complete cluster fk, cause some simple tasks just take 800 lines of code, and in the end the drivers don't even use the complexity graphics API's force upon you.
Improved this is not an accomplishment ?
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The cost/compromise is dropping support for outdated GPUs.