Their most popular song (as proven by the 10th anniversary live poll), Alleluia, will be NOTHING without KEIKO. Their most performed live song, Ongaku, will be NOTHING without KEIKO.
I don't think finding an alto to replace her will solve this problem. Fans have adapted to the sound of KEIKO being the glue that holds WAKANA's and HIKARU's vocals together. Having voices melt together as one is difficult, and will be tough to mimick the effect these three had going on for ten years (well, since sprinter/ARIA). It's awkward enough for me to hear HIKARU partake in oblivious, Kimiga hikarini kaeteyuku, and Kizuato, because she simply wasn't planned as a vocalist for these songs (I said awkward, not that I dislike it. I happen to enjoy her presence LIVE. But paradoxically somehow still doesn't feel official to me).
I'm glad they kept Kalafina going with WAKANA and HIKARU though because it opens up opportunities to see them live as "Kalafina" and to perform "Kalafina" songs. But new members? Might as well rebrand. Honestly don't know who would want to take up the role of KEIKO's replacement and be held at extremely high expectations. Prospectively, they might do well. But retrospectively in Kalafina's discography, it'll be hard for this new member to truly understand the Kajiura sound.