Not insane, but possibly suboptimal. The CPU and GPU increases in the 2016 and 2017 models where rather forgettable (in the 5-10% range I believe), but if everything that we currently believe about what the 2018 models are about to get is true, then we're looking at a 40% or possibly even higher CPU performance increase compared to the 2017 models (according to pretty much all the leaked benchmarks), and even more so when comparing them to the 2015 models. The 2015 models are still fine right now but this is a big leap when we talk about future proofing. And that's without even taking all the other improvements of the 2016 and 2017 models into consideration (much better speakers, improved screen that can get brighter and has a larger color space, faster SSDs, T1/(possibly) T2 chips for more security, Touch ID, much thinner, lighter and therefore more portable which is another factor you mentioned is important to you, and so on).
If it's a reasonable decision would also largely come down to what you're paying for it. If you're getting a great deal on the 2015 one and are paying, for example, less than half of the price of what the respective 2018 models will cost, that would sound like a solid deal. But if we're only talking about a couple hundred $/€ price difference for a device that you would probably use for many years to come, then I'd rather swallow that additional cost now than potentially having to upgrade a few years earlier which might easily compensate the savings you'd make with the 2015 model right now.