Someone, did a post a few days ago showing that in terms of cash purchases - Ethereum cash purchases is equivalent to the next 19 main crypto-currencies put together.
Assuming, that post is correct - after Ethereum goes to POS, it will gradually overtake Bitcoin.
Minimal electricity and hardware costs, plus greater cash purchases equals a growing market cap.
However, it will actually depend on the volume of ICOs on the Ethereum Network - since they do effectively sell down Ethereum in the short run, but this will be less of issue when the POW has ended.
Let's face facts, some miners are addicted to buying GPUs, they will sell down everything, so long as they can buy some more GPUs! LOL
Yeah that someone may be me...
The fact that fiat investors goes mainly toward ETH is indeed a good indicator for us but I'm not sure that this is enough per se.
It may be interesting to check how this indicator went during the past few months. Just to be sure it isn't just some sort of South Korean hype...
Plus I'm quite amazed that if so many fiats are going into ETH, the market cap isn't already bigger than BTC's. But I didn't do the maths: maybe it's just because ETH are generated much faster than BTC.
EDIT: I did my own graph that you can see below
You can see that there is a thiner gap that on the post on Reddit between BTC's volume and ETH's (that's because the 19th of July was a huge day for ETH volume).
Given that the total ETH daily trading volume is about 750 M$, you can also see that ETH volume is still concentrated in fiat vs. ETH trading on a few exchanges. On the contrary, given that the total BTC daily trading volume is around 950 M$, there is much more small BTC vs fiat exchanges or BTC vs crypto trading.
Thus being said, I found out that for each ETH mined, there are 45 kETH traded. On the same time, for each BTC mined, there are 550 kBTC traded. That explains why even if more fiat are invested in ETH, the market cap is still way behind BTC's. That's being very logical given that you create almost 150 ETH every 10 minutes when you mine only 12.5 BTC.