I've always thought that a lot of the allure of the current FM is that it's really like three bands in one. The styles of Stevie, Lindsey and Christine are so different that if you don't like one, you might like one of the others. My preference is Stevie, but Christine and Lindsey have their own set of followers. The old FM was very popular, but I think the inclusion of the Americans helped them in the US. Also, they went from blues to pop when Stevie and Lindsey joined and that also helped in their success. Blues is more obscure and selective than pop and FM's style of music in 1975 was really timely. I wouldn't take anything away from the old FM because Peter Green was greatness, but I don't know how far they would have gone in terms of top ten hits if Stevie and Lindsey hadn't joined. Would they have even thought to switch their format/genre to pop? Probably not. They would have continued doing the blues and their success would have remained selective.
I think now people associate FM with Stevie. Lindsey, too, but not as much. I don't think they can go backwards. An FM without Stevie Nicks? Really iffy. Therefore, I think Stevie is necessary to FM's future success, if they should decide to continue at all. The SYW tour did pretty well without Christine, but I'll bet they lost a few fans, meaning that when some concert-goers found out Christine wasn't with them, they decided to pass. When I bought SYW less than 24 hours after it had been released, I bought it used. There were already a few copies that had been bought and returned and put in the used section, but played like new. They had obviously been played only once. That was really telling to me. Without Stevie, I think I would have found a lot more used copies in that bin.
Hard to say about Lindsey..