Re: your first question, you can hire freelance writers through any number of channels (e.g., Craigslist, Guru, etc.) If you are looking for one in a specific industry that requires expert knowledge, this may mean you have to look harder, or post a job offering yourself. Any writer worth his/her salt can do a blog post.
As for your second question, the method with which pages are archived and linked to on a blog depends entirely on the blog. Perhaps one the following points relates to what you read:
1. Older posts may no longer be linked to directly from the homepage/main blog page, and may therefore seem not be getting as much PageRank/Authority/whatever flowing to them. However, a blog with a smart site architecture and internal linking should be relatively well-protected against this (and hopefully the posts have some external links as well!).
2. Some blogs that accept masses of guest posts shuffle them to an archived netherworld, because they simply can't handle the glut of "content" they are posting. If this is the case, the link may not be worth the time and money you are putting into getting it anyway.
A little detective work could tell you how well-linked those older posts are, and whether they are getting any juice from the main site. Focus on blogs that seem like they get some actual traffic (from people other than people looking to guest post) and you should be fine.
So, at best what you've read is a gross over-generalization. While it is important to diversify your link portfolio beyond solely guest blogging for a number of reasons (looks more natural, scalability, diminishing returns, the possible eventuality of Google devaluing byline links), it can still be a worthwhile tactic for link/brand/whatever building.