I like to keep it simple. I also happen to be a bit old-fashioned. I say that the ultimate gain is to write well. Write quality. People are attracted to good writing and behemoths like Barnes and Noble and Borders, which are still very strong companies, help to prove that. People will flock to good writing, and the better writing you can do, the more people will not only flock to it, but will COME BACK. Gruel may work for some folks, but it just isn't where I think the real opportunities are. Focus on content and let the cards fall where they may...appropriately and properly.
Don't forget that Adsense is not the Holy Grail either. If you write well, network well, and persevere, much more REAL lucrative and lasting opportunities may well come your way.
Like in investing you should never put all of your eggs in one basket, you should never seek one source as THE source. The Internet has many fads. Adsense may be one that you find one day doesn't pay dividends beyond its lifespan...so focus on something that is a never dying thing.
Again, focus on the writing. Good writing will always have a place in the world. I am certain that good writing will also far outlive places like even HubPages, so it's something worth focusing heavily on. It's something that will stay with you beyond the life of the outlets and the fads.
As for Google and SEO? I'm certain there is someone out there, born yet or not, who will come along and create something new and exciting which will change the entire course once more in this field. So don't put too much into it. You'll just have to eventually start all over again.