Unless you're expecting a Brodeur or Lundqvist, 3-4 years of quality goaltending is definitely not a stop-gap. I'd take the first four years of Marc-Andre Fleury's time in Pittsburgh (this year was his fifth; if he's jettisoned now, the Pens had a good run with him). Goalie turnover is a part of hockey; very few stay elite for more than a handful of years. If anyone would be an example of a stop-gap, though, it'd be Backstrom; possibly one year of strong goaltending left and questionable at best every year afterwards.
I think getting an aging goalie for the final 3-4 years is the definition of a stop-gap, no? I mean, I understand that turnover is a part of the game. That is a no-brainer. But you shouldn't be looking for a guy that can give you 3-4 years, then hope that in 3-4 years you have someone else available that can give you 3-4 years. If you are, that is a stop-gap, not a long-term solution. Isn't that what the Flyers have been doing (with the exception of Bryz) since Hextall left? Stop-gap after stop-gap? I know Luongo is better than Mason. But Luongo would certainly be a stop-gap. If he remains at his level for the next three to four years and then retires, who takes over? Stolarz? Maybe if he is ready and up to the task. If not, who then? Aren't these questions the ones that you try to avoid by looking for a guy that can give you at least 3-4 years, but hopefully a ton more?
Again, I am not saying getting Luongo is a bad idea necessarily, only that it will mean we are essentially writing off Mason and will be once again looking for a goalie in a couple years.
Mason needs time to get his game back, if he can. I don't want to see the Flyers fall behind the pack using that time with Mason as a starter. I think he's better suited to have a reduced role; you seem to believe that a reduced role will hamper his ability to reclaim his form. (Barring a trade) There is nobody that the Flyers can sign, other than Luongo (possibly), that would be clear #1 for the Flyers at the start of the 13/14 season, so we're likely going to see Mason as the #1 (barf) or see Mason as part of a tandem.
I don't think it will necessarily hurt his development, I think it could skew and complicate things. For instance, he played 7 games this year for the Flyers and looked awesome. Obviously he will get more than 7 games this year, even if he is a backup barring any injury. But if the Flyers want to see what they have in him, I think a 20-25 game sample may not be the best starting point. Say he plays well in those 20-25 games and Luongo plays well in the other. Mason is on a one year deal. Now we have Luongo for two more years. Do we re-sign him or were the 20-25 games a fluke? What if he looks good but not great in those games? Was that a fluke? Will he even re-sign with us to be a backup for a couple years? It simply complicates things. If you don't see that, then I don't know what to tell you.
And once again, you have to understand that I am not saying Mason is the answer or that the Flyers should "force it" and just run with him. I am simply saying that if the Flyers think he can be that guy and want to see if they are right, they can't do it by making him the backup for one year and signing a better guy for four years. If they do that, it is Bob 2.0. He will either not be re-signed or he will be traded (which I wouldn't necessarily be against...this all started because I said if they want to give Mason a chance they have to go all in and give him that chance...not half ass it).