Ancelotti or Tuchel would’ve been perfect for starting a new project. But I think none of the other coaches beside Gracia and Montella were ready to take a limping Sevilla in an ongoing season. A strong reason for that is the high requirement for this second half of the season to be considered successful for the new coach.
A successful second part would mean, quarters of CL or winning CDR or 4th place in La Liga. Anything else and we could’ve done with Berizzo until the end of the season, technically speaking.
However, if, which is a big IF, Montella still fails to reach any of the ‘success parameters’ but at least shows signs of huge improvement in the team play and promising a good future (2018-19 season), then perhaps that would also be considered a success. Otherwise, if no signs of improvement and failing to reach any of the objectives, it would be good to sack him in June and bring a serious coach that will carry on a meaningful long-term project (not Gracia, rather Tuchel or similar).