It has nothing to do with the nm in the chips, it's all about reward and availability
The fabrication process invokes moore's law.
A smaller nm semiconductor process gives you a higher hash rate and lower electricity consumption. Have you ever wondered how new generations of ASICs produce greater hash rates @ decreased energy consumption? Its due to the downsizing in nm scaling.
This observation holds true with both ASICs and GPUs. It plays a big factor in how competitive GPUs are with ASICs in terms of mining profitability.
Availability and backlog of orders might be a valid point. That hasn't been an issue in a long time though. Its more of a historical myth that hasn't necessarily reflected the condition of markets in awhile.
That is what happens in theory, but as I've told you before, the reality looks different
This is the ETH hashrate, as you can see from June 2017 to February 2018 is has jumped x5 times, and from March it hasn't advanced even 20%.
This is BTC hashrate:
From June to March it has gone up 4 times, but.....it has also gone 2.5 since then, unlike ETH.
So, what's the normal conclusion?
That is a far shorter time span ETH miners have managed to bring the hash rate closer to a point where it makes little sense to mine, this due to the availability of miners.
In BTC case, it was never a problem of having cheap electricity and mining not being profitable, and this is the reason we're having continuous growth as more and more ASICs are being produced and sold.
If indeed your assumption of ASICs taking over from GPUs in ETH mining would be real we would have seen a spike in the hashrate, right? Which is not there!!!!!