Yeah, and it's far from a sure thing. It's not as if we've poached Arsenal's best youngster or anything. These signings don't necessarily work if you take them on their own.
The point is to be constantly bringing in a steady stream of talent at this level, cheaply, factoring position and timing in a sensible way (i.e. Martinez is our only serious left-footed CB, he's 27 and Heaven is 18, so if Heaven turns out to be a real prospect, that lines up really nicely so that he'd be maturing just right to start to step up when Martinez starts to slow down.)
And if they come into the first team a bit but don't turn out to be top-tier quality, you sell them for a modest profit.