I see what you mean but a forged one piece isn't strictly bent, it's made that shape. The cranks on mine are forged BTW. The "grain" structure follows the curves and it's strong.
Yes things have moved on, but how many people do you know who could build their own monocoque? I certainly don't have the kit to do it, but I can cut, fishmouth, fit and braze a spaceframe. Brazed framea still work very well and are pretty damned light, ever heard of Caterham? Ever see how fast the R500 got round the top gear test track. Pretty impressive for heavy, outdated technology.
A welded spaceframe wouldn't be any stronger than a properly brazed frame, indeed a brazed joint will have a higher tensile strength than the steel that makes up either side of the joint. I've seen brazed frames that have gone head on into armco at speed and the joints haven't failed even when the tubing has. It's good stuff, but doing it properly takes more skill than fusion welding. Fusion welding can bridge a badly fitted joint with brazing your joints need to be perfectly fitted and that's the main reason you don't see it too much. In top level racing carbon composite tubs are where it's at. At lower level fusion welding, TIG and (please no!) MIG rule. But where you need a spaceframe and manhours aren't a problem, bronze welding is still where it's at. In bicycle frames you hear a lot of talk about fillet brazing, but trust me it isn't any stronger than ordinary brazing. It's just a fashion thing.