How do I mount curtain rods securely into crumbling plasterboard?
Stop using plugs in plaster.
These anchors are designed for drywall, and even then may not be useful for hanging curtain rods.

If you're using an anchor where the drywall/plaster is weakened, the drywall/plaster will fail and the anchor will fall out.  If you're using an anchor too close to the edge of a sheet/seam/cutout, the drywall/plaster will fail and the anchor will fall out.
All drywall anchors and plugs are meant to be used in the middle of an undamaged sheet of drywall. They are also not very good at handling varying loads, such as those caused by opening/closing curtains (not to mention little Johnny tugging at the drapes, and Felix the cat climbing the curtains).
Wood Frame
If it's a wood frame wall, aim for the studs or the header. Use appropriately sized screws, directly into framing members.
Masonry
If it's a masonry wall, epoxy appropriate plugs into the masonry behind the plaster.