It's not irrelevant how many goals a striker scores. And for the record, he doesn't just score goals. He has excellent hold-up play, fantastic off the ball movement and drags defenders all over, opening up a lot of space for his team mates. Last season, he also created the same amount of big chances as Garnacho, Rashford and more than Isak or Kudus, so it is not like he doesn't provide at all either. Maybe they didn't improve much in particular with him, but their robotic like system allows them to play identical football even without particular players. Only the absence of Rodri seems to affect them. They played brilliantly without Gundogan too, he was still one of the best midfielders in the world when they won the treble.
And let's say we bought him, or even Real Madrid. You don't think we - or they - would score much more goals?