This is fine, I see your logic. You prefer to invest into the best possible specs to get the highest resale value in the future. This makes total sense with cars or houses, IMO. However, realistically any tech (iPhones and iPads) are fast ageing consumables and Apple wants them that way, else they would be losing sales. Three years from now both Air 4 and Pro 11 M1 will be worth very little (old screen tech, not enough RAM, not fast enough, etc., etc.) If you keep it in mint condition over these years, you may be lucky to sell it for about a 1/4 of what you had payed for it. Take this example - how much would you be willing to pay for an iPad Pro 10.5” today, knowing you could also get a Pro 11” from 2018, 2020 & 2021? See what I mean? With Pro you are just paying extra for the latest tech, effectively subsidising it for future “non-Pro” products.
I expect the Pro 11 with 8GB RAM will be able to cope better than the Air 4 with 4GB RAM three years from now. Chances are Apple may not even bump the RAM on entry level and Air to 6-8GB until then. We also haven't really been seeing the same CPU gains we used to have in the earlier days so devices don't get obsolete as quickly. To be honest, I expect both iPads (heck, even the 8th gen) to perform more or less fine 5 years from now. However, the Pro model is likely to age better and experience fewer hiccups.
Yes, you are paying extra for the latest tech on the Pro. However, you're also benefiting from that latest tech now instead of having to wait for it to trickle down. iPad Pros have had 4GB RAM since 2015 and it only made it to the mid-range Air last year. Also quad speakers since 2015 as well and the Air 4's just got stereo landscape. There's been 4 generations of the basic $329 iPad and they all still have the non-laminated display. ProMotion's been around since 2017 but Apple seems content to let that be a Pro-only feature.
Mind, I think any of the currently sold iPad models are all good options. It just depends on one's budget and what features one wants. Even the 2018 (A12X/4GB) and 2020 (A12Z/6GB RAM) iPad Pros are also good options with the right discount.