I think you are right. ETH is losing value at every fork of BTC which is gaining in value. It is best i think to buy BTC now. And after the fork buy ETH or buy ETH maybe 3 days before the fork. ETH will surely be cheaper compare to BTC.
Compare to dollars, ETH doesn't lose value, but compare to BTC, it lose a lot.
Are there any dangers around it?
well, as you can see bitcoin is rising pretty fast these days and with each step other altcoins specially ethereum are getting dumped harder and harder. and best case scenario is that they go down against bitcoin.
for example ETH is not worth 0.046
BTC and a while ago it was worth 0.07
BTC and not so long ago it was 0.14
BTC.
this means if you sold 10 ETH a while ago you could have gotten 1.4
BTC but now you are getting 0.46
BTC ($9100 versus $2990) that is what you are getting now.
by the time fork happens you may get 0.04 or if you are unlucky there may be a bigger dump and you get 0.03 per coin.
this is the "danger" you forgot to see.