I sold my Apple refurbished 2018 11 inch iPad Pro that I bought in November 2019 for $250 CAD less than I paid for it (a 21.7% depreciation compared to the current-day market depreciation of 42%) Upgrading to the 2021. I took a loss of $296 CAD (or 8.67 hours of work). After the money I got from selling it, it cost me another $374.30 CAD to buy the 2021 model. So the cost of my new iPad Pro 2021 was basically $670.30 CAD. Worth it.
Next year, when the 11 inch gets the screen update (and I’m not expecting much more from it), the depreciation is expected to be closer to 20% rather than the annual 14%, making the total depreciation 62% for the 2018 iPad Pro, or possibly worse.
The 2018 iPad Pro is still great and I could have kept using it, but the resale market is much less forgiving. Even the iPad Air 2 from 2014 still has iPadOS support but probably won’t get iPadOS 15. So you can push an iPad to 6 years, but the best usability/resellability ratio is reached at about the three to four-year mark.