The original Voyager budget was $865 million (~$5 billion today), and has paid out even more for maintenance and operation since. Spend that much money on an iPhone and not only will updates be available for decades, but someone from Apple will come and personally install the updates for you.
If you only pay $1,000 for an iPhone, you are going to get $1,000 worth of support. Don't be such a cheapskate if you want greater service, I guess?
Instead, iPhones are like the basic bitch of devices and are mass produced to the point they are ubiquitous and still 99.999% of use of the device is pointless drivel.
So my point stands - if Voyager ran on an iPhone it would get those updates.
There is no incentive for Apple to provide longer term support because nobody is ponying up the money to see that support happen. People cheap out and are only willing to pay for short term support – probably because they know that they can afford to buy a whole new, better phone (most likely for less than longer term support would cost!) again in a few years, so they don't care to brunt the cost of longer term support.
NASA knows they can't afford to buy a new Voyager every 5 years – no amount of money can place a new one to replace what is out there that far out in space – so they are buying long term support instead.
Your point only stands if NASA paid Apple enough to see them agree to update an iPhone for decades or more. Simply buying a $1,000 iPhone off the shelf and sticking it into a Voyager probe would not make Apple care or change their updating tune. And if that is your point, what is the point? You would be just restating what was originally said, which seems rather pointless.