Having other cryptocurrency wouldn't cause Bitcoin to be any cheaper it will even make bit more expensive because the demand will be even higher than the supply so it's even better there are other altcoins where investors with smaller amounts can always invest in and still be profitable but they aren't as solid as Bitcoin because it's block chain is what most of other cryptocurrency rely on for their transactions as it's like the mother of them all.

Personally I don't think with Bitcoin there's any competition with others coins, Bitcoin is independent on its own and other coins are literally following suit so there's almost no competition and if there is I think it's only amongst the altcoins and never with Bitcoin, although some of the altcoins try so hard to be like Bitcoin , having block chain of their own and community, just like ETH with the ERC block chain, tron with TRC but then they all still remain as altcoins and are not like Bitcoin although they do have solid projects and community behind them but Bitcoin still stands out. Buyers decision most times on cryptocurrency depends on how much knowledge they have and that's why regardless of the many altcoins and their appreciative index some investors still stick to Bitcoin because it still stands tall it only those with little or amateur knowledge about cryptocurrency would continually be indecisive about purchasing Bitcoin and that's why you can see an investor having having other altcoins and still insist on add Bitcoin but strict Bitcoin investors hardly look into altcoins.