The downside of employee protections is that it makes companies more reluctant to hire. And also once hired, the salary to be offered is reduced to cover the estimated current value of the future employee protections. These are real downsides, particularly for young people who are trying to get a foot into the corporate ladder.
I have even personally witnessed a situation where a company evaluated a legal change to increase employee protections, and decided to layoff almost everyone in a particular country before the new law took effect. (The company was Pictage, the country was Argentina, and this was a bit over a dozen years ago.)
So I'll take the up front downsides of at will over the hidden downsides of employee protections.