It had plenty of chance of working out. I still think Manny can be a good SS in the future. And if he can't, then I doubt he will remain as elite at 3B as he's been in the past.
Criticizing it as stupid with no hope of working out is entirely Monday morning quarterbacking, even if you were against it ahead of time. It wasn't implausible at all that Manny would make a good SS. Certainly reasonable to think he would be better than Beckham, which is all it would've take to be the right move, IMO.
We've been over this same ground before. Baseball analytics and decision making evolve around assessing the probability of success. In my opinion the probability of success here was low just considering the basic analytics. Then you have the obvious issue with adjustment times that both players were gong to have to overcome (there is more including some more subjective analysis, but lets just stop at those two). Just breaking down those two basic factors the decision making comes off as poor to me. So, "no hope" and "low probability" are different assessments. I think there's also an opinion/bias here that Beckham was just a horrible defensive SS last year. He wasn't not just in my opinion but in the opinion of the metrics.
As far as Machado's future at SS. What he did this year certainly doesn't define him. It provides some evidence. He can improve and probably did improve as the season went along. I doubt he'll ever be above average, or consistently above average going forward, but he may not be one of the worst SS's in the league either if that's the position he ends up playing in the future. Great arm but I don't see any other notable SS skills there (range, quickness, athleticism etc.) and I don't see the greatest work ethic either.