Saying the S6 has "the best battery life of all flagships" is just a way of saying "the least poor of all the poor performers." Smartphone battery life, across the board is frankly dismal compared to what is needed. Ever since the advent of the modern smartphone, we've been conditioned to think that a workday off the charger and a night ON the charger, EVERY DAY is acceptable, as is carrying around a wall-wart charger and gord with which you will tether yourself to a power outlet mid-travels. We have done a fine job of forgetting just a few years back when most folks had flip-phones that could go a couple of days before needing a recharge. (All of which, BTW, had replaceable batteries.)
Yes, I get it. The smartphones of today are very powerful, and very busy machines, and necessarily will draw power far in excess of the dedicated phones of just a few years ago. And, the technology of Lithium-ion batteries hasn't changed appreciably since that time, so large-capacity batteries are necessarily off the table for a thin, powerful device.
Which is why I argue for the replaceable battery. It's awful hard to view a phone as "wireless" if it has to be attached to a wire half of the time. And, a replaceable battery is an efficient and highly portable way to get decent phone power, regardless of outlet or cord availability, and you simply don't have to wait to get a usable charge when you swap a battery out. 30 seconds or a minute of downtime, and, bang! full charge. Not that it's any more convenient than a portable charger (from a practical process perspective) to remember to keep a spare charged and slip one in your pocket on the way out the door, but it's certainly not any LESS convenient, either, and a replaceable battery is often smaller and lighter than its portable-charger counterparts.
Hey, maybe ubiquitous Qi mats will supplant the need for higher charge capacities... Or maybe somebody will make a graphene breakthrough. Until then, I'll pine for my replaceable battery.
mjc