The capability was one reason, financial and technical, but the environmental concern and research was another reason that made that 50 years. Removing the limitation of the first reason, it may not take 50 years to decide, but a long time is still needed make sure that we don't make a big mistake as we did to 三门峡水利工程. While 三门峡水利工程 was amended by 小浪底水利枢纽, the "red flag" if done is many times bigger, so a mistake there will be many times bigger.Thank you for these additional details.
I don't think "red flag" can only be done 50 years from now though. The South-North Water project took 50 years because China was not capable of carrying it out earlier. It's different now. I don't think it will happen fast, but I don't think it will take another 50 years either.
Desert reclamation is happening even now, and China is deploying a variety of technologies in pursuit of that goal. Some will inevitably fail, but others may yield promising results. Maybe it's not as grand as what "red flag" is promising, but bit by bit, the Gobi desert is turning into a grassland.
I support project like this, but the bigger it is, the higher risk of mistake, the more dangerous that risk will lead.