It seems many people are speculating the rise of BTC and therefore are transfering value from alts to BTC. But if you want BTC, buy it with FIAT! Fresh new funds are needed on the market! You are dumping alts making prices dump hard and maybe are speculating you will be able to buy more of the alts later on, but what idiot will selll it to you at that dumped price?? So if you want BTC, buy it with FIAT or at least don't expect to buy back more of the alts for cheap!
You definitely raised a valid point because the use of Fiat to purchase BTC instead of Alts will guarantee inflow of fresh funds into the market.
However, many in this market are speculative just as the nature of the market is so the drive to always want to maintain and secure the already earned profits makes them enter and exit into BTC each time there is fall in altcoin prices.