Yeah they sort of got replaced by the newest adversary turned ally. Tien was Goku's rival but was replaced by Piccolo Jr who in turn was replaced by Vegeta.

The characters got that strong that the lesser characters weren't able to keep up at all and just became useless and they were written out of it a little.

Krillin still had a good role though until the Buu arc.

With this new movie Goku has now got a new rival in Bills and is two transformations infront of Vegeta who could be the next to become useless.

Why does the screw attack clip say Goku cant breath in space? I understand it shows him underwater against Frieza but at the same time Goku's father flew out of Planet Vegetas atmosphere and fought hundreds of Friezas goons no problem while mounting an attack on Friezas ship.

I've only read that saiyans age more slowly but have the same relative lifespan as humans. This is backed up by Daizenshuu (Dragonball guide books) 

Is there anywhere that Akira Toriyama says they live forever?

It's from either Vegeta mentioned it in DBZ during Android or Buu saga, OR Vegeta mentioned it in Dragonball GT.

But in DBZ if you are strong enough you will never die of natural causes, and if you are stronger, then you will never age old, as shown with Piccolo & Tien.

http://www.youtube.com/watch?v=TLgcDh2NDTg&list=PLvx1taOOcI0fsaBY9BCpNJXHgBF4b8HJ7

6:30

"We Saiyans evolved as a warrior race, we retain our youthfulness so we can fight longer."

I personally dont believe that means that the do not age, just that they age much more slowly to a rapid decline. 

Also, Goku, who is much stronger than Tien, almost died to a heart desiese which is technically a natural cause of death.

Piccolo is a Namekian who has a MUCH longer lifespan that most races. The Guru elder was approximetaly 500 years old at his time of death. Kami, if you include the time that he was "the nameless soldier" when he had not seperated with King Piccolo, would have been approximetaly 700+ years old. So Piccolo has plenty of youth left in him which has nothing to do with his power level.

And in Dragonball Online(which was completely controlled creativley by Akira Toriyama and is stated to be the continuation of DBZ) which takes place 200 years in the future, All of the Z warriors are long gone. Roshi is still alive since he drank the immortality elixer from his (ironically) now dead immortal pheonix.

So, thats why I dont think they live forever.

"Why does the screw attack clip say Goku cant breath in space?"

Because he can't breathe in space. In the new film they also appear to be fighting in space but it's just one of those weird inconsistencies. Perhaps the oxygen in the atmosphere reaches farther than it does in our real life world.

"Is there anywhere that Akira Toriyama says they live forever?"

They don't live forever, all that was said is that saiyans stay in their prime longer.

"But in DBZ if you are strong enough you will never die of natural causes, and if you are stronger, then you will never age old, as shown with Piccolo & Tien."

That's not really true, it's just the way it was drawn really. The looks didn't change all that much over the series is all. Goku died of a heart disease also in one timeline. Piccolo was supposed to have a long lifespan anyway.

People also forgot how young Piccolo is. He's 4 years older than Gohan.

Yeah that's right, that was always weird how he grew up so fast yet Dende didn't at all.

It was also confusing on what Piccolo Jr was to King Piccolo. At times he referred to him as his father and other times his reincarnation. He did however have all of King Piccolos memories and was able to fuse with Kami to become the Namek they used to be previously so I guess it's the latter.

I suppose if you take the mind into consideration then Piccolo is probably hundreds of years old but his new Piccolo Jr body wasnt much older than Gohan no.