Well, the designers don't seem to have shown any understanding of the concept that certain systems don't work well for certain genres-- Call of Cthulhu being the most blatant and offensive example of a game that doesn't fit d20's "default assumptions."
So, yeah, I'd say it was designed as a system to replace all the others. It doesn't work as a replacement for several other games, so I don't think it will supplant other systems. On the other hand, D&D is the most popular roleplaying game, and several incredibly good d20 products are coming out.
Since the games I prefer to play work well with d20 mechanics, all is good.