Moore's law was famously exponential, until it wasn't, because there are physical limits. Grain per hectare made some noticeable jumps, but no growth or even negative growth due to organics. Limit now is probably what the ecosystem will bear, and that is something linear or constant.

The reason why I think a polynomial is the best we can do is that some very special processes generate exponential behaviour, mostly stuff that is self-replicating. I don't see those in technology. Technology doesn't reproduce all by itself, it needs to be invented, financed, etc. Those are limits that force it subexponential.