Yeah, I'm sure you're right. That said, I still encounter people who haven't decided. The day before our caucus I had lunch with a teacher who still wasn't sure between Hillary or Bernie. I don't know how many people there are like her, but probably not enough to have a big impact. At this point, the Dem primary is more of a turnout game. But for Trump, I suspect there's basically nobody who's undecided, unless they just woke up from a twenty year coma. Whatever small percentage of people there are who haven't made up their mind between Rubio and Cruz, or between Clinton or Sanders, there's gotta be even less who are on the fence about Trump. He's too polarizing.
Really, most people's vote is determined long before the candidates are even announced. You choose based on who you already are.