The team has been built to win now, as the sun on Melo's career is setting. This plan is obviously not working. So there are two alternatives:
1) Decide that the team is still stacked with talent to win now, just needs proper motivation to play defense. Then it is on the coach, so Hornacek must go, and someone who can motivate players needs to take over. Apart from Phil himself, there is no one like that.
2) Admit that the personnel is simply not there (Lee not the defensive stalwart we hoped for, Noah really looking done for the career, Melo no longer able to carry a team on his own, KP not ready to do it yet). In which case, the quicker we blow up the team, the better. Trade Melo to a contender, trade Noah for expiring contracts, tank for a top 3-5 pick, and see what the FA market brings next season. Then let Hornacek hire his own coaching staff, assemble his own team, and run his own system to have some cohesion and continuity. And hope the losing does not get imbibed into KP's psyche.