Of course. This is the same with all Garmin watches past and present but has never been a problem. Should a battery go after 5-6 years Garmin will almost certainly replace it with a re-furbished model for a very good price. They were still doing this until recently 10 years after the Forerunner 205 was introduced. I'd say Garmin are pretty good at helping people out with failures on old watches.
I successfully replaced the battery on my 305 and have done a few for friends. Garmin will do it as well; their charge to "change a battery" out of warranty is typically around $60, which is almost-certainly just a swap for a refurb rather than an actual replacement.
If you can find the correct battery doing it on the Fenix will probably be easier than on the 305, which involves cutting the seal on the case (the Fenix is screwed together.) The risk of opening one of these is that you lose whatever watertight integrity it had.
This is a very carefully controlled and engineered reality that we the consumers must face. It is called "planned obsolescence". It is wrapped up in a variety of strategies depending on the industry.
With tech gadgets, its the battery. Have you noticed that the majority of cell phone manufacturers no longer produce cell phones with user replaceable batteries? It is ideal for manufacturers to produce an incredible product, at a premium price that must be tossed and replaced entirely in 5 years or less.
Video gaming systems? Oh you have to replace the entire system every few years as games get more complex.
Major home appliances are now engineered to only last an average of 7 years. Plan on replacing your washer, dryer, fridge, stove, etc every 7 years on average.
It's the same with software. From Operating Systems, Applications and hardware platforms (laptops, desktops). It is no coincidence that software gets more complex, needs more powerful hardware. If you don't upgrade your software you risk incompatibility in exchanging files with other users. It's a vicious cycle that puts the hardware and software manufacturers in a great place to take your money about every 5 years.
The only way to beat the cycle of planned obsolescence is to be satisfied with "older" technology and to learn how to fix/repair/maintain things. Whether it be a major appliance whose control board just "failed" after 7 years. Or be willing to find the battery and figure out how to replace it on the cheap after 5 years so you're not on the hook for another $700.
Even if you ARE willing to do so in 5 years when the battery in your Fenix gives up, Garmin will almost certainly have rolled out new tech that only a newer device will work with. Maybe it's a new revision to ANT+ or a new version of Bluetooth. They have us. We are to blame if we keep pursuing the latest and greatest tech. I assure you that if you were in charge of marketing, product development and income generation at one of these major tech companies you would do the EXACT SAME THING to your customers.
Think about it differently and be willing to do what you can to extend the life of your tech, be willing to live with older features. My primary vehicle is a 2003 Subaru Forester with nearly 250k miles on the clock. I debated buying a new one in spring of 2016. I couldn't stomach the thought of car payments ever again in my life. I also could not tolerate spending nearly $4,000 to replace the head gaskets, timing belt, water pump, spark plugs, etc.
So I watched some YouTube videos. I spent a full weekend and about $400 in parts and did it myself. I hope to get another 10 years/100k miles out of that sweet old machine. I also just bought and installed a modest $110 Bluetooth Kenwood Stereo. I haven't had car payments since 2005!
I know some people think this is important, but I literally couldn't care less. I have the 5x, I have seen Garmin's estimates that the battery will last 5 to 6 years, so I'm assuming I will get 4 out of it.
Whatever Fenix is out at that point will almost certainly be enough of a draw for me to replace this watch. Tech moves too quickly.
Plus, if it is an issue, some clever person will figure it out.
Lithium chemistry batteries have roughly 500 cycles of life in them when made before they're deemed "unsuitable" for use, assuming you get an "average" cell from the run.
"Unsuitable" means 20% capacity loss.
There are two qualifiers on this -- the first is that TIME is also a factor. Lithium chemistry cells lose a couple percent a year in capacity just due to time. But cycles are far more important to battery life; every cycle does a small amount of damage to the cell internally, and there's nothing you can do about it. A "cycle" is one excursion from "empty" to "full" and back; if you charge at 50% depletion then you used half a cycle.
This is where a watch like the Fenix wins BIG over the typical "smartwatch" that has to literally be charged DAILY. That's more like a phone, which is why your phone is good for ~1-2 years and then the battery is toast. Same for most smartwatches.
The Fenix, on the other hand, only has to be charged every 5-7 days for most people. So will you get an "average" of 5ish years before the capacity loss bothers you enough to be unhappy?
Actual battery life span may vary from climate, how you are using it, how often you charge/recharge, how you charge/recharge, etc.
I am not a heavy user and I don't wear my Fenix 24/7, I only wear it for my activities or as my travel watch (then i would wear it 24/7). My last Fenix 3 was 3 yr old when I sold it and I don't notice much battery degrading at all.
I expect the same level of battery condition for my 5X, if it maintain more or less the same battery condition after 3 years, I would be a happy camper.
A lot of devices / appliances with rechargeable battery are not user serviceable. So I don't ding Garmin for the battery.
