I don't really see the comparison (although it's about their breakup albums, more than as individual artists), but I think a few people here are Alanis fans, and she's quoted extensively in this article.

http://entertainment.msnbc.msn.com/_news/2012/06/14/12219593-is-adeles-21-this-generations-jagged-little-pill?lite

Jagged Little Pill was such a phenomenon that it meant Alanis's entire career was going to measured and defined by it and to many people nothing was ever going to come close to it again for her, which meant in the long run, it hurt her commercial and mainstream career.

21 could very well have the same negative impact on Adele's career in the long-run, no matter how good her next few albums are.

These albums had a life of their own.

I feel that way too. I think that Alanis was such a unique artist that it may have been the only time, right place right time, she could have had such success. Adele is a little more generic, so she may have more legs.

I think another thing worth adding is that many listeners praise Adele's vocals and hold her to a higher standard than they ever held Alanis. So even if her future works don't amount to 21, I think she will still find success in that. It's a shame that Alanis is not as huge as she once was because so many of her later albums were great works!

I think she's the white Mary J. Blige.