Well mainly it's because they have targeted the 2017 season as the point we are going to be competing for the playoffs. And if we don't make any FA moves and have our younger pitchers sorting things out as you say (and I agree with that part, they will be having major ups and downs the next two seasons as they find their level) then we won't be competing for the playoffs (particularly if we wind up trading Julio for a bat, which seems likely given all the smoke about him being traded lately).
So their stated claim to not sign majors FAs doesn't jive with their claim to have a a solid team in place by 2017 for the new stadium. One or the other will wind up being wrong. Which is why many of us don't buy it completely.
And personally once we made the trades this offseason I figured we'd suck for the next 2 1/2 years at least, so it's not a big deal to me if we let all the youngsters play and get another high draft pick next year. I'd much rather see that than trying really hard just to have a .500 record. Nothing is worse for a franchise than consistently being average.