This time Barcelona will have a harder season because Real Madrid gut much better players and they are trying hard to improve their performance for the next season. However, the Barcelona we saw in the last season with Xavi, got enough strength to even race for the champions league.
Although Real Madrid has better players, I doubt they will perform well if Real Madrid fail to sign Mbappe. I know Real Madrid already signed Joselu, but I think his quality isn't enough to compete with Barcelona. If we compare Lewandowski quality and Joselu, everyone must be aware that Lewandowski is 1 level above Joselu.
When Lewandowski left Bayern Munich for Barcelona some people were the thing he won't have the same performance in Barcelona also since he is an old player, they were thinking buying Lewandowski was not a smart move at all. While the fact about Lewandowski is he just needs some time to adapt himself to his new team. However, I think since he getting old, Barcelona should start seeking another forward soon.
Lewandowski still performed with his best, people who doubted him may not know him well. Although Lewandowski is no longer a young player, it is not a problem as long as he is still productive. We know that Ronaldo still played well when he is in the same age with Lewandowski. I think Lewandowski can play well in the next 2 years.
If Mbappe really like to play for Real Madrid he will definitely join his season in this season or the next season and he will not accept the offer from PSG to extend his contract even if they pay him more money for the next contract. But I think Real Madrid should think about an Alternative option for Mbappe.
I think Mbappe can move to Real Madrid if he doesn't extend his contract with PSG. I'm sure PSG will try to sell him as soon as possible in the current transfer window. They already try to get his replacement, you must hear that PSG is trying to sign Harry Kane. This is a sign that Mabppe may leave PSG in the near future.