But technically, the onus is on the buyer to prove that the fault existed at the time of purchase if making a claim after 6 months, thus making it more problematic (have to obtain engineers report etc).
That is not the case. You do not always have to prove a fault. There is also how long you can reasonably expect the item to last before failure.
If you spend £20 on an own brand kettle and it dies within 18 months no one will lose any sleep. However if you spend £80 on a Kettle from Miele or Bosch and it dies in the same period then you'd have a good chance of either getting it replaced or some compnesation from the manufacturer.
So 3 years for a £2000 - £3000 laptop/desktop is nothing. Would imagine you could get them over a barrel upto 5 years for a laptop/iMac and probably the full 7 for a Mac Pro.