The offside line is really set by the goalkeeper

A high defensive line is not a decision the defenders can make on their own, and clubs still recruit as though it were.

Ask why a team defends with a high line and you will be told about pressing, about compactness, about squeezing the pitch. All of that is true and it skips the person who decides whether the line is survivable.

The goalkeeper.

A back line pushing up to the halfway line is leaving a huge area of grass behind it. Somebody has to be responsible for the ball played into that area, and it cannot be the defenders, because they are facing the wrong way and starting from a standstill against a forward who is already running. So the space is either covered by the goalkeeper or it is not covered at all. That is the whole calculation, and everything else is decoration.

Which means the ceiling on your defensive line is set by how far from goal your keeper is willing and able to operate, how quickly he reads a through ball, and whether he can clear it under pressure with either foot. Give a squeezing back four a keeper who is superb on his line and unhappy outside the box and the line will drop, no matter what the coach demands, because defenders are not fools and they can feel who is behind them.

I think this is badly reflected in how goalkeepers are assessed. The public numbers are dominated by shot stopping, which measures the moments when the defence has already been beaten, and by distribution, which measures a skill in possession. The sweeping contribution barely appears. Defensive actions outside the area exist as a statistic but it is a low-count, noisy measure, and it does not distinguish the keeper who makes six interventions from the one who never needs to because his positioning has already deterred the pass.

There is a nice indirect test available, though. Look at where the opposition's forward passes stop. Against a genuinely aggressive sweeper, opponents stop attempting balls in behind after about twenty minutes and switch to playing in front, which registers as a change in their passing profile rather than as anything in the goalkeeper's column.

He has not made a save. He has removed a category of attack from the match.

That is worth more than most of the saves, and it is priced at roughly nothing.