I think that depends on what you do with the "old" device. If you can get a good price for it then the incremental price of the upgrade is not so bad. Also, it depends how heavily you use the devices. If, like me, you make your living from using these devices for 8+hrs a day then sometimes having the latest device is worth that cost to gain a few percent productivity. Even more so, if you are a software developer, as I am, then you need to have the latest device so develop software for.
I'd say the smartest financial approach is to buy a popular model (eg. not lowest storage tier or weird colour) and look after it carefully and then sell it on while it is ideally still under warranty (~ 2years) and hopefully looking pristine. If you don't need the device on release day then buy from the Apple refurbished store to save another 10-15% on initial purchase without dramatically harming the resale value.
I do agree that the iPad Pro will last a good few years but with the way Apple is continuing to advance CPU and now GPU performance along with various other features, a 4 or 5 year old iPad will definitely start to lose its shine.