As the commish of a world with no trading (though only in season 3), I can say that many find it fun and challenging to compete in a world where you don't have the advantage of trading. I've never had any trouble filling open spots (though we've only had 2 offseasons). Most openings came from folks saying, "I tried it out, but it's just not for me."
With the commish gone after the first 2 seasons, you get a little insight into why "For Life" may have trouble keeping owners.
I wanted a world with no trading mainly to eliminate trade raping, which is the biggest cause of worlds becoming lop-sided. I know, vetoes eliminate those trades, but try getting 10 people to agree on what should and shouldn't be vetoed. So getting rid of trading is limiting enough for teams, I couldn't imagine throwing out free agency, rule 5, and waiver wire claims. Add in MWR and a player payroll budget restraint (to ensure some attention to organic player development), and it's a great world.