It's not just the "we", it's also the "when".
The tools that the world runs best on were built over a period of man-years that were not mythical either.
Plus if you go back far enough, you hit a point where often the key people involved were 10x more suitable than the average or below-average candidates who would be most likely to come under consideration today.
To have the most realistic chance of success, you just have to allocate 10x the resources that you think it would take on first blush. Especially the amount of calendar time before deployment. Simple ;)
Plus if the world in question is really already running without the replacement tool, when are you willing for the world to come to a halt during a pitstop for the proven tools to be retired while the new replacement is inaugurated?
Otherwise, the most talented and productive tool-building team in the world would further need a much more-rare capability that was not even required of the original builders, the knack for servicing airplane engines in flight.
I figure that would be when.