How long should you expect backlinks to stay valid?

I was reviewing the TIER 1 links from one of the SEO packages I bought about 6 months ago. I checked them about a month after buying that package and most of them where still indexed.

I did not check on them again till today. It seems 97-99 percent of them are not indexed (none of the EDU or WIKI). And only a handful of the ones that are indexed are still valid pages (when you click on them the page has been deleted or account suspended).

The TIER 1 links included in this particular package were EDU, WiKI, Web 2.0, BookMarks, Article Submissions, ETC.

My question is this, how long should you expect these kind of links to stay valid for (indexed and active)? Is what I explained above to be expected?

Is there anyway to make these links stay alive longer? They had back-links going to them from TIER 2 sites (same problem with TIER 2 by the way)

What are some types of links that last forever or at least longer than a year.

Is this type of SEO just good for the short term, with a need to be constantly replacing the "dying" links?

Look forward to hearing some input on this!

No matter which type of platform you are using, you need to use quality content.
Spun content wont get indexed and you can try your luck with some indexing services. Regarding link building, you need to build new links time to time. Web 2.0s stick for really long time if they are done right.