Not all of them necessarily wanted the morphing power, though. I'm sure there were some of them who were happy with what they were, got along with their host, and wanted to keep that way of life. If the host had no problem with it, then why should the Yeerk become a nothlit?
I find it unfair that the Yeerks only had two options at the end, as far as we were told. Surrender and be transformed, or become prisoners? Not very great options, really. I mean, I know that they just lost a war, but as has been stated multiple times on this forum AND in the books, not ALL the Yeerks were bad, or had a choice about whether they were in the war or not.