The split had more to do with GCN being very compute heavy and AMD's software completely failing to take advantage of that. GCN got transitioned into CDNA, and got heavily reworked for graphics as RDNA. This is why while RDNA is nowhere near as good at compute oriented features, it tends to outperform NVIDIA in pure rasterization.

In contrast, current NVIDIA GPUs are very compute heavy architectures (to the point that they often artificially limit certain aspects of their consumer cards other than fp64 to not interfere with the workstation cards), and they take great advantage of it all with their various features combining software and hardware capabilities.

You can't really isolate crypto mining. If a gaming card is profitable to mine with, miners will buy it and use the gaming drivers.

Correct. Whether or not a card has official GPGPU support is irrelevant for its use in mining, you can use pure Direct3D or OpenGL for mining if you really want it. The decision to not support customer GPGPU by AMD only hampers other useful non-mining applications.

Ironically, mining is probably the most optimized GPGPU application a regular end-user can run on AMD GPUs. One reason was that GCN was a computing-first GPU, but also that miners have a strong motivation to go through the troubles of overcoming the lack of computing support by AMD. They would write the entire application in OpenCL 1.0 and optimize it to extract the last bit of performance in spite of its difficulties - the hashing kernels used for mining also tends to be much simpler than other applications, so it's feasible to do so. Meanwhile other people who want a decent programming environment would've picked CUDA without looking back.

if i remember correctly, AMD GPU cards was being sold very well to crypto miners. The split wasn't because that. They did, because gamming and computing having different hardware optimization requisites, and that allowed to get better and more competitive GPUs and computing cards on the market.