I typically update my electronic equipment when the vendor no longer provides firmware or OS software updates for a product. My experience is that for an iPhone or iPad, iOS updates can be expected for 3-4 years, for macOS, Apple will provide updates for Macs that are up to 7-8 years old. I bought my current iMac in 2013, but it runs macOS Mojave just fine. After a battery replacement, my iPhone 7 runs iOS 12.2 just fine. Apple released the 11” iPad Pro in 2015, so I think my current iPad Pro will be able to run iOS 13 just fine.
Fuji has been good about updates to the Fujifilm X-T2 even adding or improving features with each firmware release. Last summer, I bought a Fujifilm X-T2 used (pre-owned) on MPB. The camera does everything I expect of it. I have just one lens on my X-T2, but I have budgeted for a new lens at the end of this year. I bought some inexpensive Fotodiox adapters to use with some vintage Pentax lenses that I already own.