Interesting points. But it's still an open question if a singularity type growth is actually likely. Just because it's technically possible doesn't mean it's something to think much about. And it sure seems to me like systems usually become harder to optimize as they approach optimality. Of course, improving an advanced AI won't be comparable to trying to optimize a near-optimal algorithm, which is a very limiting approach. But it also won't be comparable to simply adding more computing power, which is a very limited approach. It will probably be somewhere in the middle.

It is interesting that human intelligence has enabled a huge advantage in capabilities over other species, yet other species have not been able to dominate each other through intelligent behavior alone (or have they? counterexamples welcome). Granted, it took a very long time for humans to gain dominion over other species. But it seems like the marginal returns to intelligence grew dramatically when human level intelligence arose. It could be that there is a trend with intelligent agents, where the returns to being slightly more intelligent become more and more decisive as agents in general become more intelligent - and this would seem to be a very basic fact about the nature of the universe, specifically relating to optimization and complexity.