This might not be a well-thought suggestions, but the idea came to my mind during shower and I'm in a hurry to get to work:

Wouldn't a flat rate of 12 points on defense loss solve this whole thing?

PoV of low rank players: they lose 12 points anyway from people 300+ VP over them
PoV of high rank players: instead of going backwards with 70% of their attacks, they just don't advance in case of a loss, which can still be countered with participation.

Yes, this would open up again that more attacks grant you higher rank, but not to the extremes we witnessed in the first three months