A miner is not a bank, your private key is your bank guard it with your life, you could have 1 cpu mining the bitcoin network it doesnt make your wallet easier to crack, but it makes it much easier for someone to perform a 51 attack (actually with 1 cpu mining the network, its very easy to perform a 51 attack as you have 100% of the network)

The only thing to change when more miners join the party is the difficulty, this changes how hard it is to mine bitcoin, so if anyone else wants to effectively mine they will need a more powerful machine

If a bank or a rich person wanted to buy a ton of ASIC miners and run them on the network they could in theory get enough hashing power to manipulate the network, perform a 51 attack, but as more powerful machines are build and sent all over the world the difficulty is constantly rising, this means that bitcoin is actually more secure as the rich person(s) who might want to break the network will require even more hashing power to even dent the network..

The current hash rate is 87,656.67 GH/s

Lets say an ASIC miner can do 80 GH/s you would need about 550 miners to perform a 51 attack, as more asics are build and sold the number required will increase


Bottom line, ASIC will not KILL bitcoin, ASIC will make Bitcoin stronger