I was mainly attempting to stress that cold wallets are more useful when the stakes are high.

A modern phone OS will have plenty of extra weaknesses when compared with a security-focussed server/desktop OS.  For phones, security (and transparency/openness) ranks far below concerns such as price, weight, size, spec., mass-appeal, ease-of-use, and being feature-rich.  I'm reminded in particular of an incident where Android-users lost bitcoins, not due to any user or app-developer error, but because Android's SecureRandom function was flawed*.

If you do ever decide to manage serious amounts of money with your own phone app then please be mindful of these extra risks.  Good luck!


*Note that this flaw was found because of Bitcoin.  Bitcoin demands much greater security than most other information systems.  Even a mobile banking app is less appealing to hackers given that profiting anonymously is more difficulty and bank transfers can be reversed.