This is a common misunderstanding of MTBF. An MTBF figure tells you nothing about the lifetime of a particular product. The MTBF figure and service lifetime are two completely different things. What the MTBF tells you is how many failures should be expected, on average, for a given number of operational hours. If you have 1,000 disks and operate them for 8,000 hours, then that's 8,000,000 operational hours. If the disks have an MTBF of 400,000 hours then you would expect about 20 failures.
However, that doesn't tell you anything about the service lifetime of the drives. You might find those MTBF numbers are only valid for the first 50,000 operational hours (a little over 5 years of continuous operation) and then the MTBF markedly worsens and almost none are still operable in 100,000 hours (by which time they will be well and truly obsolete of course).
There are lots of things wrong with MTBFs and the stats from disk manufacturers. They won't generally tell you service lives (which they probably won't know anyway. as they'd only accelerated ageing to test) and, as some studies have found, batch-to-batch variations can be enormous.
But people should never equate MTBF with service life - they are completely different things.