As Southgate rightly said: "He's an all-round centre forward. I've always liked his general play but it's the goals that are making him another level of player." - that's spot on. He's never going to belt in goals left, right and centre against the top six, but he's going to be a massive asset to the side in those games and he's going to get goals at a fair tick. 10-15 a season.
It wasn't just that Lukaku didn't score; it's that he didn't do anything in those games, and even in your average game he was missing nearly all of it before popping up for 10 seconds to score. DCL is the utter opposite of that, because DCL is a far, far, farrrrr better footballer than Lukaku - it's not even close - and will therefore be an asset throughout the entire game.
And no, you don't get more points for a goal against a top six sides - but you win trophies by scoring them when in an elite level side, and it's what you should be doing as a striker for one of those sides. Lukaku doesn't do it. Hence United bombing him out.
If you put DCL in that Inter side instead of Lukaku, he'd score 10 goals fewer a season, but he'd score 3-4 more important goals, the team would score more goals overall in those games too and Inter would get closer to the title as a result, because he'd be turning up in the "six pointers" at the top end of the table. I can't explain the difference any better than that.