That may be fine for some, but newsletters are inherently limited by the amount of space they are willing to provide to links, and by their domain, because most letters are topic-specific. While newsletters can support maybe 20 links per week, RSS is adept at handling hundreds if not thousands.

Moreover, one of the great benefits of RSS is that it is customizable and one's reading experience pertains specifically and only to them. With newsletters, one is throwing their reading experience to the mercy of the newsletter editor. This is not to say that newsletters aren't valuable. Their spontaneity and conciseness provides a great reading experience. (Although, I think Twitter is superior at this type of thing anyway.)

But, given the idiosyncrasies of newsletters, I just don't believe that they are tenable alternatives to RSS. Those who want to read a lot of content from an eclectic array of specific sources still need something like RSS.