I partially agree. The Dyson vacuum mechanical parts might be buy for life, while the batteries specifically do only last five-ish years. I don't mind that, because I hate having to vacuum with a cord more than I hate having to buy and replace a battery on a multi-year interval. I also like not having to buy bags, and only having to occasionally rinse the filters.

Except, bags to batteries, I would take a guess that the battery is cheaper than the bags over a five year period. And more importantly to the question of BIFL, since the battery is easily purchased and replaced, why would that make it less of a lifelong purchase than something like a flashlight which needs disposable batteries, or a vacuum needing replacement bags? It's not like the battery is locked in, as with smart phones, and designed not to be replaced.

Buy the genuine Dyson battery, don't cheap out. The really cheap, Amazon, batteries don't last as long, nor provide as much oomf. The better aftermarket batteries presumably have better life, and do have better power, but it's still less than a genuine Dyson battery.