I believe manufacturer’s don’t think pros bet their livelihood on a $400 piece of gear.
By "not user-replaceable," I don't mean "You can take the top off, unsolder the backup battery, and solder in a new one or at least a battery holder with a couple AA batteries or a 2032 until the proper replacement shows up from Batteries+." I mean like you can't take the thing apart, unless you have special tools or a portable electronics workbench.
It's impractical to have backups of every piece of gear with batteries that can't be replaced. Besides, if you bought the backup around the same time, the battery in the backup might not hold a charge either - unless you were meticulous about recharging it periodically, remembering to let it go to half-charge before extended periods of non-use, and then recharging it on occasion whether you needed it or not. It's also important that battery quality is variable, some might go on for years and some might not be able to hold a charge after three months. It doesn't do much good to have a backup if it has the same issue as what it's backing up.
But I was thinking mostly of consumer equipment. No, I'm not going to buy a $1000 backup iPhone because the battery might go haywire when I'm not near an Apple store, or away from home on a tight schedule. I bought a backup Android
for a couple hundred bucks with a replaceable battery. Swap sim cards...done. And then there's this B.S.
I do consider something like this
at least marginally replaceable. I'm sure anyone would rather find someone like me who knows how to solder and massage cases apart than get charged $150 for a factory battery replacement.
Remember where all this started (from Wikipedia): "iPod batteries are not designed to be removed or replaced by the user...compounding the problem, Apple initially would not replace worn-out batteries. The official policy was that the customer should buy a refurbished replacement iPod, at a cost almost equivalent to a brand new one."
Since then, it's been okay to use batteries to hold customers hostage to high factory replacement fees, or sometimes sketchy third-party solutions. Not a fan...this is why in my reviews, if a battery is not-user-replaceable, it gets dinged. I don't care if the case has to be 1/32nd of a inch bigger.