If UFA starts at age 31, and say there`s only one quality goalie available, then that guy`s agent starts playing the teams against one another ( i.e. my guy wants to sign with your team, but you`re only offering $5M while this other team is offering $6M), it becomes a bidding war and the goalie ends up with more than he`s worth. That`s what`s happened the past few seasons.
If UFA starts at age 27, there`s likely to be more than a few quality goalies available any given year, then the teams start playing the goalies against each other (i.e. "why should we pay you $5M when this other guy is just as good and we can sign him for $4M.")
Anyone with half a brain can figure this out. Of course, since the average NHL owner has less than that, it`s taken them a lot longer. Lowering the age for UFAs benefits players who want to get out of the city they`re playing in, other than that it benefits the owners. So the owners, even though they won`t mind lowering it for the above reasons; will act like it`s a huge concession and demand something huge in return. The NHLPA won`t fall for it.