I don't understand this. If 0.8 fixed the bug that 0.7 contains, shouldn't encourage everyone to upgrade instead of downgrade? And it's fine that the longest blockchain is on 0.8; that is the latest version. If you're not patching your software, I am not so surprised that you're vulnerable (in this case to double-spending attacks).

A second thing that I don't understand is why 0.8 people are vulnerable to double-spending attacks. It wasn't said that 0.7 was safe, but it was said that 0.8 was vulnerable. But with two separate blockchains, wouldn't both be vulnerable? Or at least 0.7 because they don't accept 0.8's blockchain?