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The last point you make is an interesting one. It's not super clear cut.In my very, very long experience in trading and buying video cards, paying a bit more actually does get you a better card. Take the spectrum of the 4-series. The worst card in the line up still works, but it runs hotter, is power limited, and the components are generally skimped on, especially for nVidia (AMD card are generally quality across the board). Paying 10% more can mean the difference between an annoyingly loud card that cant't be overclocked, and one that does what you want. That said, the benefits depreciate on the upper end e.g. a TUF is exceedingly close to a Strix in all areas.
On such high end cards as the 5090, I bet that in 18-24 months, the Astral/Suprim/Master cards will sell for at least £200 more than an entry level cards (e.g. Palit GameRock). So whilst it may be £500-600 more now, it's a bit less than that in terms of the total cost of ownership if you upgrade each gen.
Ps, don't hold to me these above figures. Your investments may go down in value.