(1) Much development is already moving from CUDA to the LLM, so less of an issue. Nvidia is also doing more work to increase interoperability. Could be an issue I guess, but doesn't seem like it since there's nothing close to CUDA or the ecosystem.
(2) AMD has attracted significant investment looking at appreciation in its market cap, with a PE ration 3X Nvidia's. However, AMD is so far behind in so many ways, I don't believe it is an investment problem, but structural. Nvidia has just been preparing for this for so long it has a temendous head start, not to mention being more focused on this. Remember AMD also competes with Intel, etc.
(3) Hyperscalers already are building their own chips. It seems even Apple used its own chips for Apple Intelligence. It's relatively (which is doing a lot of lifting in this sentence because it's all HARD) not too hard to make custom chips for AI. The hard (near impossible) thing is making the cutting edge chips. And the cutting edge chips are what the OpenAIs of the world demand for training, but releasing the newest best model 1-3 months ahead of a competitors is worth so much.
If anything, I'd say the biggest threat to Nvidia in the next 1-3 years is an issue with TSMC or some new paradigm that makes Nvidia's approach suboptimal.