The corroded sections do not look like parts of the subframe to me. They look like the subframe to bodyshell mounting points. Typically these areas are made from heavier gauge steel to the rest of the bodyshell and would need to be replaced like for like. The final picture shows that the corrosion has eaten right through both sides and the bottom of the box section. A proper repair would need the engine and subframe to be removed, and the mounting points to be remade and accurately aligned- very difficult and time consuming work.
The degree of corrosion is such that the subframe has almost certainly already moved out of alignment, and a bumpy road could well cause it to completely detach, in short it is in a very dangerous condition.
Victor, are these the front mounting points? I think they are. I have repaired mounting points on cars before, and it not a nice or quick job. The metro VDP in my avatar has had both the rear subframe mounts rebuilt by myself, they were a notorious failure point back in the days when they were common. I thought corrosion such as the OP has shown was almost a thing of the past outside of Russia.
YouTube channel "Garage33" and "AutoCraft" (Russian) have many videos of severe corrosion damaged cars being repaired. It's the sort of thing that used to be done 40 years ago in the UK, but apart from rare, classic or much loved cars is rarely attempted now.