MTBF is what you need to take a look at for a mechanical system, if you are concerned about service intervals (in a HA cluster style setup, where you can handle fixing after break down, without downtime), and for e.g. continuous operation of optical disc and magnetic tape drives, as those wear out over time (though even laptop BD-R drives reach 1 year active spindle MTBF). There is determines how often you have to go to the system and swap faulty drives, to have at least x% working, and over how much time you can budget the CAPEX.

Of course this breaks down at an MTBF of over 50 years, as thosre rarely mention exotic failure modes, and don't actually have an MTBF of 50+ years over the life, but an annualized failure rate corresponding to 50+ years MTBF, measured over the first couple of years or even the first year. For non-wet-electrolytic-capacitor-using computing, one can calculate a temperature-dependent MTBF in the 5-25 years range, mostly depending on how bad the chips are hit by electromigration and similar aging in the semiconductors. This is incidentally a reason why I miss clock speeds for different processors as reported by overclockers to at least in some cases extrapolate the life due to electromigration, as there is a formula with like iirc 2 parameters, which gives a temperature (and maybe voltage) dependent lifetime/MTBF for this semiconductor device. I'd likely sttrive for about 5 years MTBF on the processor, if speed is of concern and reliability/uptime not in the foreground.

> For a MTBF of 2 million hours, that means that on average, if you have one thousand drives, then you should expect one drive failure every 2000 hours

That assumes your drives fail with a constant independent probability, like nuclear decay events (Poisson distribution). The reality is more like https://en.wikipedia.org/wiki/Bathtub_curve .

MTBF is not a good metric for complex systems used under wildly-varying load conditions, but ... it's a metric.