I think the likes of Milner, Lallana etc are ok supplementing quality, they play some good football but collectively don't contribute enough in terms of goals and assists, but if you have quality around some of them, they're more than capable players.
Look at how Henderson benefited from the presence of Sturridge and Coutinho a couple of years ago, he got among the goals with his intelligent runs from midfield.
I don't think it's a case of "they can all go as far as I'm concerned", that sort of outlook is waaaay OTT, if you add a couple of big players to this squad, notably a quality defender, midfielder and attacker, then some of these lads will thrive with the pressure off them to weigh in. It's difficult to chip in when you don't have the quality around you to occupy defenders and to provide a genuine threat that the opposition have to concentrate on. When the onus is on you and you're not a big goalscorer, then you're likely to buckle. You're more likely to contribute when your team has a couple of guaranteed match winners. We're never going to have a team of match winners, you need a few players with real quality and the right tactics, so we're halfway there at least. United on the other hand have quite a few big potential matchwinners, but their attack is starved of oxygen by their nullifying tactics. That's a contrast worth thinking about, our style can make lesser players look crucial at times. Adding quality to that can only help Klopp realise his vision.
I'm as disappointed as anyone, but you have to be longterm-ist about this, we just don't have a couple of the necessary tools to push on yet, but it's not really constructive to say rip it all up and start again, we don't need to do that at all, we need a spine, which we have done since the days of Reina, Xabi, Torres, etc. I also think that Firmino is starting to show his quality, despite him not showing himself to be that regular a goalscorer yet, if you get a quick goalscorer alongside him, then I think he'll be excellent. And as said, Can is turning into some player.