You're missing the game theory behind what keeps bitcoin secure, which is probably why you're so convinced that it will fail.  For example, you said a new entrant wouldn't have enough hashrate to compete.  However, if it requires a lot of hash rate for that new entrant to compete, then that means the system is very likely not centralized.  The reason is that if the miners formed a cartel (and bitcoin became centralized), then they would all lower their hashrates to earn a greater profit.  But by doing so, they would invite competition from new members that aren't in the cartel (b/c suddenly it's profitable for new miners to join).  

TLDR: I believe bitcoin will remain decentralized if it remains possible for participants to freely join the network and mine (i.e., it remains a dynamic-membership multi-party signature (DMMS) consensus scheme).