what about instead of pure pos, a hybrid like pow/pos? It is still not free from problems but, at least it won't take the whole mining activity out of the question
Yes, that is what I had in mind. Keeping PoW and adding PoS.
I like the idea of adding TaPoS as long as it doesn't interfere with bitcoins planned disinflation and PoW security model. You do no need consensus or a fork to make this happen either. Simply create a TaPoS/PoS blockchain and add functionality to a bitcoin wallet where the distribution of the PoS tokens mirrors BTC distribution. These TaPoS tokens could be used as another security layer and to provide 1 second confirmations.
Within your wallet the the TaPoS tokens would be hidden and non trade able for usability and so this isn't a competing alt.
I.E... The user receives a payment and the wallet immediately starts confirming the transaction with TaPoS every second and than after approximately 10 minutes the regular PoW confirmation comes in with the wallet showing a distinction between types of confirmations. Thus in a decentralized manner you can get a higher degree of confidence that a cup of coffee you sold is secure but with more expensive items you could wait till 1-3 PoW confirmations came through ontop of the TaPoS confirmations.
Added benefit - This could be a nice warning that a 51% attack is occurring and allow the seller time to hold off on the transaction when the TaPoS confirmations are failing but the PoW confirmations are being approved.