I wouldn't think that Lukaku will be back at his peak because I think he's been past that in the previous period. I agree that Romelu Lukaku can still score goals and contribute to Inter, but he won't be as good as before.
I think so too and the peak of Lukaku's career has passed in the previous few seasons, although Lukaku is still in good form and still able to score goals for the next few seasons. Another consideration is age that is no longer young and other factors due to frequent injuries.
Remember that Lukaku is already 30 years old and will soon be 31 in May. His pace has dropped, his goal stats are no better, but he could still be a threat to any defender Inter will face next season. So even if Inter manage to keep hold of Lukaku next season, then maybe they will finish the deal at the end of the season without needing to buy him permanently.
For the next season Lukaku can still contribute to Inter Milan, even though his goal statistics are not as good as in previous seasons. Lukaku also has an advantage in aerial balls and his header will be a threat to opponents, but the problem is that Inter Milan cannot fully rely on him alone because if he gets injured it will affect Inter Milan. So the option of buying another striker must also be considered because after all Inter Milan will play in many competitions next season.