I agree with you mostly although I don't know whether Messi is still thinking in long-term project terms. I think he wants a competitive squad and in a best case add one more Champions League title to his record history. But I have seen that Lewandowski said he wants to play with Messi. I gave the idea some thought and Barcelona has to be careful again because they want to rejuvenate the team now and start a new era, but let's not forget that Lewandowski is going to turn 35 in a few months and Messi 36. Busquets is going to turn 35 as well. This is not the end of their career necessarily as we can see with Modric, but they still need to consider their decisions well in Barcelona.
Lionel Messi popularity give beneficial for Barcelona without as long term project due he has 36 years old, but he can bring positive impact with jersey sold more than expected because all fans still hope Messi will back to Barcelona next season, there are not problem between Lewandowski and Lionel Messi because each other have difference position.
Messi will play as second striker and Lewandowski as center forward, but not easy if want to signing Lionel Messi because he has offering higher salary with PSG for extended contract and Barcelona face problem with financial right now. But if Messi return to Barcelona will make La Liga have more competitive and excited again.