Tuesday's 2-2 draw was only Blues' second point in five home matches, ending a run of three straight 2-1 defeats at St Andrew's.
But a furious Rowett gave his players a post-match tongue-lashing.
"I can't tell you some of the words I used in the dressing room," he said.
"Short of me saying we were spineless, I've gone pretty tough on them."
Top scorer Clayton Donaldson's brace had looked like earning Blues a first home win since beating ex-boss Steve Bruce's promotion-chasing Hull City on 3 March, but North End's 89th-minute equaliser completed their revival.
"To be 2-0 up at home and end up drawing the game 2-2, I can't accept that lack of desire," Rowett told BBC WM.
"It looked to me like the game was won and I think my players thought it was too. We started thinking we were on on the beach, making bad decisions and not defending when we need to defend.
"If that becomes a norm, then we could become a lower-half team. We've got to be better than that."
The former Burton manager continued: "It's a lack of professionalism to see games out. We've got to be more disciplined. It probably sums up our season from March onwards.
"If people play like that, they won't be here next season playing in my team. We know we've got to improve next year and bring new players in."
Blues remain ninth in the table, but they are now 10 points adrift of sixth-placed Sheffield Wednesday, having won just three of their last 15 league games.
With just three matches left to play, Tuesday's failure to win finally confirmed that they will spend a sixth successive season in the second tier of English football.
Gary Rowett was talking to BBC WM's Adam Bridge.
Birmingham City manager Gary Rowett admitted he was left "frustrated, angry and disappointed" by his side's lack of discipline in failing to hold onto a two-goal lead against Preston.