When people are commenting on the upcoming redesign in 2019/2020 - what are the expectations? Is it expected that there will be a major redesign from the ground up both externally and internally?
I am a bit skeptical that we will definitely see a major external redesign in 2019/2020. I think Apple will instead choose to continue to make small tweaks with no major changes to the external design. For example, I could see upgraded internals, upgraded display technology (and thinner bezels), continued improvements to the keyboard, maybe drop the touchbar, etc. From an external perspective however, I think they've hit on the pinnacle design they've been moving towards for the last 15+ years and I don't see them abandoning it so soon with a radical change.
Historically, redesigns every ~4 years made sense when the Mac was a much larger portion of Apple's business, but as it is increasingly becoming a much much smaller chunk, I don't think it would make sense for Apple to devote resources to redesigns so frequently. Not only would that take away from other higher priority segments of the business, but as we've seen from this current line, every time you do a redesign it introduces new bugs. The longer you stay on a design, the more those bugs get ironed out. Each machine sold becomes more profitable - the R&D cost is spread out further and further, manufacturing costs go down as efficiencies are gained, and Apple loses less money as machines need to be repaired less frequently within the first year.
I hope the changes we would see in 2019/2020 are similar to what happened with the iMac Pro. The iMac has had the same case/shell for 6-7 years, and there is no sign that it is changing anytime soon (if they had plans for it, I would think they would've done so with the iMac Pro). Instead, when they released the pro, they focused on internal changes and introduced the new cooling system. The hardware engineering teams didn't have to redesign the entire machine - they just focused on the internals and from what I've seen it has been a success.