If they have someone standing around because there's no line at the checkpoint I'm not going object.
While the time ahead people show up will cause some variation it's going to be a pretty small factor compared to the numbers from looking at the manifests. They won't be spot on but it will average out. They can't get it perfect on the time it will take to screen people, either, but it will average out.
You're assuming a fictional world where TSA can magically vary its number of employees on a minute by minute basis depending on need. They work in 8 hour shifts, and your not going to get many competent people if you force them to alter their work hours on a daily basis, so that means roughly stable shifts. When there are multiple up and down surges over and 8 hour period, it isn't plausible to have all the workers you need for each up surge without having twice what you need for most of the down periods, even if you try to stagger the start and end of shifts and breaks. There are only two options: 1) increase costs and waste by erring on the side of being able to cover all the surges, or 2) increase wait times by erring on the side of reducing costs and waste during the slow times.
The TSA has been choosing #2, most likely because of pressures from "small government" conservatives and the airlines to keep costs low. But a merely forceful will of the public could make that change in favor of option #1. Given that a private contractor would not realistically have any competition, why would the choose to lower profits under option #1. They wouldn't, and the contractors at European airports don't because European government tightly controls everything these contactors do (aka, no "free' market).
BTW, here is an article published in 2001, just after 9/11 and just after congress voted to put TSA in control. It notes how private companies in the US used very poorly trained minimum wage ($6 per hour) workers as screeeners, almost all who quit within a year or two, compared to Europe where screeners got middle class salaries, 4 times the required training hours and far less job turnover? At Boston Logan (where 2 of the highjacked planes came from), they experienced full staff turnover every 6 months in the years leading up to 9/11.
The biggest difference is that in the US, the private companies contracted with the airlines themselves (aka "free-market"), whereas in Europe they had to contract with the government who had strict control and oversight over what they did, who they hired, what they got paid, etc..
