Ultimate you're paying for the odd's something will last a certain amount of time, but a more expensive device won't always outlast a cheapo one.
Ended up buying a cheapo fridge because its only being used for mostly drinks in a different part of the house, a better fridge will cool quicker, keep the temp around all food similar and be safer, even if it lasts the same amount of time there is also how well it does its job in the meantime.
However with Miele I'd say two things, IF you get it with a 10 year warranty, some of their stuff comes with it, others you need to buy during offers to get an upgraded warranty for free, then I'd say worth it. I'm under the impression(might be wrong) that Miele don't do parts nor release repair manuals to anyone so you get stuck with expensive parts through authorised repairers, where with other brands anyone can get the manual/order parts, and get it fixed more easily by more people. So outside of warranty Miele's can be expensive to fix and get parts for.
Realistically without an awesome warranty its a gamble, £200 fridge, or £1000 fridge, if the £200 fridge ends up lasting for 3 years and the £1000 for 15 years, it ends up cheaper spending £1k upfront. Do cheap POS goods sometimes last over 15 years, sure, do top notch parts sometimes die after 3 years when the warranty is up, sure. Its a gamble, the odds are on a Miele lasting longer than a Beko and could be worth 3-4 times as much.
Overall buying something middle of the road from somewhere that will do a 5 year warranty probably ends up being the best gamble. But as above, that Miele might work better over the time it lasts.
From my experience of parents stuff, Armada fridge has lasted 15 years, one breakdown after 12 years, easy/quick/cheap fix, worked for 3 years since with no problems. Miele dishwasher, works fine, isn't massively quiet, isn't a great internal design, bad shelves that are ackward to fill, but no problem for several years and came with 10 year warranty. Miele washing machine, worked for ages, no problems, pretty quiet, much quieter than old John Lewis machine that broke down loads but was always fixed for free, inconvinient though.
If you can get a Miele with 10 year warranty for no extra cost, do it, for 10 years a quality device that will be fixed/replaced is likely worth it. 3 year warranty, if it dies after a few years you've paid way more and not had good value for money.