No need to justify how you spend you money. But yeah I suppose if you are going to use it that long, I can see why you may be willing to pay for so much
I personally never keep a monitor more than 3 years, once warranty is gone I get rid of it and buy new tech. Plus I enjoy buying new tech and trying different things out. I think £500-600 is plenty of money for them to make a nice profit on a Freesync 2 4K monitor.
Well ok I pulled the number 10 out of thin air, but the sentiment is I'd want it to last a long time. I generally feel that's the best way to approach most purchasing / investment decisions. Spending more on something of greater quality that will last a long time usually results in you spending less in the long run and having better products than spending less on lower-quality products that don't last as long. I'd rather buy a great £2k monitor once than buy 4 £700 monitors.