I can think of one barrier: there aren't that many easy-to-use services
that allow you to convert fiat to altcoin X (without using btc as an
intermediatory). Also if OpenBazaar decides to switch
to an alt, that does not yet mean much until there are actual people
using it.
I've done some research on darknet marketplaces. I remember seeing
only one where you could also pay with an alt (Dash) but I don't think
that option was much used.
In a word, I don't see Bitcoin being displaced soon, despite the development
woes. And this is coming from someone who tends to be somewhat
more sympathetic towards altcoins than most of you are.