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the welds they show seem fragile
The forces trying to pull two clamshell halves apart halfway along a box member are not the same as the forces trying to rip a tube clear of its moorings at a junction. It's mostly shear, and the weld is much longer. Stacks of dimes not required.
Strictly speaking Cubs have a tube joining the headstock to the rest of the frame, so they're not fully pressed, but the bigger and faster 1950/60s Benlys were pressed clamshells, as were the Dax monkey bikes among many others. Even the mighty Yamaha R1 is just a bunch of aluminium pressings welded together along the edges 🙂
Ask a Honda Cub owner. OK, the Cub and previous pedal cycles welded together from pressings have been steel, but the car industry has been building frames this way from aluminium for a while now.