This would require a modular design where the CPU resides on some kind of exchangeable module. While this is not 100% impossible to do it requires a much more complex hardware design which we currently do not aim for. A modular approach also requires more space since you need extra connectors to connect the modules. We will not have this space since we use up any possible space by separating things (baseband, GPS, CPU etc.). Integrated designs with mobile+WiFi+GNSS inside the same chip package as the CPU save a lot of space/room, which we can not 
So we will end up with bigger device anyway and we do not want to make it even bigger by adding the complexity of modules.
Though I fully understand your concern and wish!
Once we have a case design (I mean a design and fit of internal parts) we can discuss with the manufacturer a possible solution so that users might be enabled to replace the mainboard by a later version, e.g. with i.MX8 and so at least reuse case, display, battery etc. I can not promise anything right now but we will surely keep this in mind.
Cheers
nicole