Confused with SEO, keyword research and Google Keyword Planner

I am trying to find an answer to what happens if all keywords in the world were to run out and there is nothing but competition left. Is it even possible for all niches to be high in competition at once? In other words, in the future, wouldn't there be nothing but competition because all the keywords are taken up? Would everyone stop marketing then?

I am also still trying to figure out exactly what I am supposed to be looking for when searching for keywords.

I used the Google Keyword Planner to generate related keyword ideas about the topic of "weight loss"

They have the "competition" and "avg monthly searches", and "suggested bid". "Ad impr. share" - I haven't seen anything with that, it stays at 0 all the time.

Then I type most keywords in Google and get results in the millions. Does that mean that those keywords are not worth it? I tried using "allintitle:" and the quotes around many of the keywords, but most of them still had a high number of results.

I bought Market Samarai a few days ago, and I am using both Google Keyword Planner and Market Samarai for keyword research. One of my books suggested using the SEOTC and SEOT as a valid indicator of a good keyword. I also installed SEO for Chrome and SEO for Firefox today to find out the PageRank for the top links.

I'm reading Mike Omar's book "How to Make Money Online -learn how to make money from home with my step-by-step plan to build a $5000 per month passive income website portfolio (of 10... each)". He says that a PageRank of higher than 4 is not worth competing for. Looking at the search results, I see that the websites with the page ranks are not in order. Like the 1st result in Google has a PR of 4 and the 2nd link has a PR of 5. Shouldn't they all be in order?

I'm confused....