- Public key to private key reversal. (I've heard that it may be possible to do that with quantum computing and pollards kangaroo method)
Possible doesn't mean it's probable.
1. Pollards kangoroo already exist, but it's not enough to brute-force private key generated with good RNG. You might able to solve private key which is part of puzzle or generated with weak RNG.
2. Quantum competing are decades away.
- Finding collisions for RIPEMD-160 hashes. Are we sure that 2160 is strong enough? What if it becomes weak in the next 20-30 years?
Let's hope SHA-256 is still strong enough in the next 20-30 years.
A change on a consensus rule confuses the people of what is the real bitcoin.
The term "real bitcoin" is ambiguous, what defines it? Some people might refer to
Principles of Bitcoin, but few people going further such as for/against big block size limit and off-chain/side-chain network.