The answer is it's not really possible to do a much better job. With unlimited time the result would not be that much more fair or effective. Firing the bottom 50% is impossible even with unlimited time to think about it. Employee performance evaluation isn't very effective, and Musk has no reason to trust Twitter's previous performance evaluations, and forcing people to work 6 months under a new performance evaluation process with the threat of a 50% layoff over their heads is not compassionate either, nor would it have a much better result at the end anyway. There is no silver bullet. Better to rip off the band-aid in one go.

You might say that a perception of fairness is important so that employees trust leadership. Well, what's likely to happen is the fired people will think it's unfair, and the people chosen to remain will think "well, at least the right choice was made in my case". Of course, the opinion of the remaining people is what matters to the company going forward.