fair point. no guarantee he would do that though, if he stays longer he may well sign another contract. i don't think we're in a position to wait, it's not like he would struggle for minutes with us, we need a RW. better for him to develop here with us alongside Martial and Rashford than stay in Dortmund, on the hope that maybe in a couple of years we can save a few quid.
It's always a possibility he'll renew, but it's an outside chance. The lure of Dortmund for these players of this level of potential is to be used s a stepping stone, not to spend their best years there. It's what Dembele and Pulisic did. It works for BVB as well, as they make a hefty profit on it. Renewing could lead to ungodly demands by BVB and pricing potential suitors away and that's not in the interest of Sancho so long he's in demand.
You make it sound like getting him early only has advantages, but it's not the case. Too much expectation and a hefty price tag on young shoulders can have quite adverse consequences for his development. And if he keeps developing according to his talent there, you have a less risky purchase; a more mature, more accomplished player, yet still very young and malleable.
If you wait too long you could lose him too though, so there's risks either way.