Assuming Xavi is still the coach, Barcelona will still be in second position at the end of each season until the club is strong enough to win Laliga, but currently that a new coach has taken over I don't know ifit will be a problem for Barcelona to finish second in the table next season. Flick is a good coach and I believe he will be supported by the management but it might be difficult for them to win Laliga till they have the players that can outsmart Real Madrid.
You have a point about maintaining the usual position in the table after changing coach, it might be difficult to maintain second position with a new coach because everything might seem different for the new coach and will probably need time to adapt so it is possible to see a fall in the performance this first year but then La Liga is not too competitive and even with a new coach it shouldn’t be difficult to maintain their current position even if they can’t win the title. I believe Flick will bring positive energy to the team and not negative energy.
We can't compare Xavi Hernandez to Hansi Flick because they're both different and possessing likely similarities. Barcelona will bounce back to formidable form, all they need is enough time to heal from the severe critical injuries that have hits the club and also affected them so badly. Barcelona president, Joan Laporta will always push for excellent results and we know his next plan for the club.
Xavi did his best and it’s unfortunate that his services are no longer needed despite winning them a league title during his stay at the club. Xavi is a good coach but also every good coach needs good players as well and to currently Madrid has better players than them and until they can keep up with the quality of players i don’t think it will be easy to win the title ahead of Madrid.