Build a vex holonomic and a vex 6 wheel and have the Vex 6 wheel play defense on it. Should sell your team pretty quickly on not holonomic.
Have you built, tested, rammed, destroyed, rebuilt, retested a swerve drive? If you have, then great. The only reason (if swerve is desired) you'd want to run holonomic over it is weight savings, simplicity, durability, at the expense of all traction versus defensive robots. If you haven't, your team's got a point in that swerve is deceptively hard.
If you want, send them videos of 1714's 2008 robot, which is holonomic. (Just don't tell them there were programming bugs
)