Count me as another who hated Peggy's blue polyester dress. I've worn polyester in exactly that gauge and thickness (choir dress -- ugh), and I will aver that it is not at all conducive to romance. And the hair! Then again, it's all period appropriate.
If the new firm name turns out to be Emerson-related, I will be full of joy. Then again, as we've seen, Peggy doesn't know her Emerson all too well.
Joan's partnership is just a 5 percent stake. She gets a say in how things go, but it's not as weighty as Don's, no matter what she would like. At the moment, she seems to be the only person who fully knows how the company operates internally, both financially and administratively, so she's extremely valuable in that respect, but what will be her share in the new firm, and what will be her role? Joan was certainly the one who had the most to lose by losing the IPO option, because her standing in the firm was dependent on the status quo.
An IPO was one way to double the company's size; the merger is another. But a merger seems far riskier, from the point of view of the less powerful partners and employees.