How do you secure coin and and your responsible to customer if hacker stole bitcoins?
Why we need coinbank while we can save everything in our own method? Just buy hardware wallet, send coin to HW, and keep your HW. As easy as that right?
So why do centralized exchanges exist today, and by the way, are you using them? It is true that using centralized services to store bitcoins is not a good idea because we have abandoned banks for bitcoin also because we do not like centralization. But centralized exchanges and services still have a large number of customers using their services. So, I think there's nothing wrong with the OP having an idea and wanting to implement it.
Perhaps you and I, many people will not use OP's services but there will be others who will as long as OP can make them satisfied and trustworthy. This is a business idea so we shouldn't dash OP's hopes just because we don't like it.