Barcelona management will not take the risk too quickly to find a replacement for Lewandowski in a financial condition like this. As long as Lewandowski can still be relied on, Barcelona management will look for a replacement player with an easier option such as a player who has free agent status. And in my opinion if Barcelona is eyeing Alexander Isak, it is just a rumor. On the one hand, Newcastle will not release the player at a cheap price and while Newcastle currently still needs the player. So at this time Barcelona can only survive with the existing players and look for a replacement player slowly without rushing which can put more pressure on Barcelona's finances.
Your points are valid and what I read about Lewandowski and Barcelona is even different from some rumours flying around, the guy's contract would be extended by Barcelona and he has been a positive addition to the club, as expected. As for Alexander Isak, Newcastle will not release him so easily and would want to make a fortune out of him. The clubs that are more seriously eying this gys are even Arsenal and Liverpool, not Barcelona at this moment.
Lewandowski's Prime is still very useful because for me, age is just a number and does not have much of an impact on Lewa decline so far. His presence in Barcelona is quite important as a senior who is able to motivate Barcelona's young generation. But looking for a replacement option is also okay as long as the transition process is well managed. For Hansi Flick, he can currently rotate players little by little because Barcelona young players are undergoing rapid development under his guidance.