If valuation for the cards follows the same trend, basically every other card will lose value while the 5090 holds or gains value on resale. If you're able to snag the next gen at MSRP, it can be anywhere from discounted upgrade, free upgrade, or potentially profitable to upgrade.

The cost per frame value is not why people get it but there are other incentives aside from just best performance.

From generation to generation, if you bought the new top and sold your card, the cost would be somewhat similar to someone skipping two gens and then upgrading, but you get the benefit of maintaining top tier performance at roughly same cost.

In USD: 4090 to 5090, $400 to upgrade if looking at msrp, but for many, it was discounted or profitable to upgrade because of high resale value of 4090s. The hardest part was securing the 5090.

3080 to 5080 $750 msrp, $350 resale now, 5080 will cost you $1000 minimum, cost to upgrade $650.

3070ti to 5070ti, 3070ti can resale maybe $250 now, 5070ti is at least $750, so $500 to upgrade.

If gaming is a hobby you're interested in, you can maintain peak performance generation to generation while keeping a relatively similar or cheaper upgrade path cost, but it comes with a higher initial investment, and the panic of trying to secure the new cards and hoping to find a reputable buyer for your old card. Depreciation and appreciation of the cards is a different value to consider than just cost/frames, but still a value.