To move to that next level you are right but I was looking at it purely from the perspective of 'can Lukaku's goals be replaced' and the answer in my opinion is yes. We obviously don't have the stature in the modern game to be able to attract another 25 goal a season man like Lewandowski or Aubameyang and therefore we will need to replace the goals with 2 or 3 players.
For the time being we have brought in Klaassen and no bids look like coming in for Barkley. That should surely be 15 goals between those 2 and there is plenty of time for us to bring in players who will contribute more goals.
I could fully understand the panic and concern if by the end of August we have lost Lukaku and haven't brought anyone in but I don't believe that will be the case. For example if we got some of the players we are linked with we would go from having a combined 35 goals from just 3 scorers Lukaku (25), Barkley (6) and Coleman (4), to close to 50 with Giroud (15), Sandro (10), Rooney (6), Barkley (6), Klaassen (6), Sigurdsson (6). That's extremely pessimistic for Rooney, Barkley, Klaassen and Sigurdsson as well given they will be playing in a very good team. Obviously Sigurdsson arriving may see the end of Barkley but we also have the likes of Bolasie and Mirallas who could replace Barkley's goals, or a signing I haven't mentioned.