You have to define what's simple. Bitcoin is very simple to me.
What tromp probably referred to is the simplicity of the system design. Make sure to check this video [1] if you're interested in the topic.
If you read the content of the linked page, you can see separate functionalities being enumerated like consensus model, emission, blockchain format, sync format, supply audit, PoW Algo etc.
If we define these as vectors and define 0 to be the simplest solution that can theoretically exist then we can, with some bias, assign a value to each dimension.
This means a blockchain can be seen as a point in a multidimensional space where (0,0,0,0,0,0,0,0,0) is the theoretically simplest possible blockchain to exist.
Of course we don't know what that looks like. But from the systems I've seen, I would agree Grin is by far the closest to that origin point and thus the closest to the theoretical optima of blockchain system simplicity.
That said, Bitcoin is closer to origin point than most of the other chains, but it's quite a bit more complex than the design linked. With regards to making Bitcoin simpler, it can be made simpler to use, but system complexity is unlikely to go away because you'll need to support all the current functionality which includes its expressiveness (scripting).
1 -
https://www.youtube.com/watch?v=LKtk3HCgTa8