I think Mother Goddess and Chimera do entirely different things, for all that they both keep the runner out.
Chimera you want multiples of, to protect a remote server at all costs until they can break absolutely anything (or parasite it out), usually working in conjunction with other ice. Ice Wall in front of Chimera, say.
Mother Goddess you can only have one of in play, and while it's uncrackable turn one (short of Femme or Inside Job) it only grows more porous as time goes on. Basically, if they can break any of your Ice, they can break Mother Goddess. It's definitely better than Rainbow, especially for NEXT, but it turns from Hard ETR to taxing ETR almost immediately.
I like it a lot, but even as a tax-ice, they can choose the cheapest breaker to use, and turning it back on is much harder than with Chimera. Its use in a regular deck feels much trickier than in NEXT, so I think most folks'll want to keep their Chimeras -- because for all that their utility decreases with time, it never quite goes away entirely.