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!
I don't see why anyone would want to use alts to buy bitcoin, you are just not getting enough value for your coins if you are doing that. If anything what you should be doing is converting bitcoin to the altcoins that you believe will end up doing good. You are right, it is better for you to buy the altcoins with fiat and later convert it to btc to cash out.