I do not think the mobile device will be a good idea to store the bitcoin block chain. It is better for the desk top.
I agree with you, although there are micro SD with large of space but still there are problem mobile devices Will be hang because of running out of source power.
And i think The fungtion of mobile devices are not for to store the bitcoins block chain, mobile devices are just for calling and sending messages.
I don't think that would be problem to create battery with higher capacity and also what abou power bankss? They can recharge your device multiple times.