Bitcoin can become centralized if no solution against this is proposed...mining difficulty increases and new ASIC hardware gets developed. It would make it less possible for any average user to mine Bitcoin, thus making it more centralized each day would still remain as it should be and im strongly believing on that thing because no matter how difficulty increases and miners would be more developed or upgraded it doest really mean that it will make bitcoin to be decentralized and we all know that bitcoin cant be hardly controlled.