A new iPhone every five years means you’ll have a harder time getting one, like whenever a new video game console gets released.
Take pre-orders a few weeks in advance, then everyone can have it day 1. No scalpers reselling them on eBay for 10 times the cost. No stampede at Apple stores because they already have a few weeks of inventory.
The accelerated product cycle is why you have awful launches like consoles. The product is barely finished and rushed into production in time for Christmas. There's not time to build the actual units. Slow down a bit and you can solve that problem.
Inventory in a warehouse does not have the kind of cost you think it does. Most of the "cost" people talk about are hypothetical profits that a different product occupying that space might generate in the same time. The real costs are insurance and warehouse leases. Per unit, it's almost nothing.
A company like Apple could easily stockpile months worth of inventory and never even notice the real physical cost of storing it.
Opportunity costs, or any other "virtual" cost, are still meaningful figures.