I bought the base 2017 iPad ($300 on sale) a couple of months ago, and I have been using it as my primary device during this time. We have a Mac Mini and MBA at home, but I don't use them very often. I decided to experiment with an iPad as a primary device when I saw the WWDC Keynote that described new iOS 11 features. When the software updates this month, I expect my iPad will become even more capable and useful.
I considered buying the iPad Pro, but it didn't seem to make sense unless I bought the ASK and Pencil to take full advantage of the Pro features. So, I was looking at $300 vs $900 ($650 + $150 + $100). That seemed like a pretty significant cost premium for an "experiment". So far, I have been happy with my decision, but I can see how others might have gone a different route.
As for longevity, it is possible that Pro users will get more life from their device, but I doubt it. Pro users are more likely to be compelled by new features, and tablets are still an evolving platform.....so, I would figure 3-4 years with either Pro or standard iPad.