Smartphone voting sounds like a recipe for disaster. The fact that votes are cast in private in an otherwise public place is part of what makes the system work. If you let a large number of people vote remotely, you have zero certainty that a) the voters weren't pressured into voting a certain way, say by a domineering parent or b) that the device wasn't compromised.
Personally I agree and am not an advocate of smartphone voting.
However, I can tell you what someone who was an advocate of smartphone voting would say.
To (a) they would say we already allow postal votes, so the pressure/vote-selling is already with us; and furthermore although the pressure/vote-selling is bad for democracy, low turnout and voter suppression like making polling stations hard to access are worse.
To (b) they would say if Apple Pay can be secure, so can online voting. Maybe even more secure! I mean, in my country the ruling party doesn't have enough members to put an activist in each polling station watching the ballot box, even if they worked a 15-hour shift without toilet breaks. So it's not like the current system offers total protection against fraud.
