Not really. Clocks are materials limited, but new instructions or increased threading (i.e., getting an i7 instead of an i5 which has been crippled by intel) can speed things up significantly.
I'd say you are over-exaggerating the definition of the word "significantly". The problem with current CPUs and their materials is about what happens with electrons, transistors and so on. You can only go so far. Other materials are better and can give somewhat of a significant difference but that doesn't necessarily mean a higher clocked CPU frequency. If you want to integrate other components on the CPU that requires room which means you need to shrink transistors again. Whatever you do, it is not easy to escape that reality.
Btw, increased threading is not going to make things faster as there are two kinds of workloads: sequential and parallel. Sequential is not going to benefit from increased threading. Increased threading also demands more from the developers. They need to change their apps in such a way it can use that increased threading. Sadly this is rather complex to do and as we've seen now from years of experience with multithreading there aren't that many apps using this properly and efficiently.
People have been saying that for 20 year and at some point that will be the case but I think there's still plenty of innovation and improvements left in what they can do 🙂
I think you misunderstood them. What they said is that this is inevitably going to happen but they didn't know when. Moore's law has been changed a couple of times because of that (they went from 12 months to 24 months some 30 years ago already!). It has been motivated by statistics as well as technology at that time. The fact that more and more factories have difficulty with die shrinks says enough (e.g. 10nm seems so difficult that Intel had to postpone it) and if you ask any researcher in nanotech they'll explain that silicon isn't very suited for that (mind you, CPUs are moving towards nanotech). As said above, if you want to do something else you quickly run into that issue. You are more looking into a complete redesign of the CPU.
I'd also skip the upgrades, but rather add the leftover money towards AppleCare
As an EU citizen I'd rather spend that money on the CPU upgrade as the AppleCare is completely useless here due to regulations. As a Dutch citizen I even have a longer warranty. On a machine with a 5K display the GPU is going to be its biggest bottleneck, hence why I'd rather upgrade the GPU than anything else.
But do people really upgrade that often? I keep my Macs for 6-10 years. I buy it, enjoy it, and focus on making money with it in my business. For me, flipping it after only 2 years in seems rather quick, but I get it.
There are some enthusiasts here that do this and it's about the only thing you can do with a Mac if you want to upgrade. Macs have never been upgradable, that's an exclusive feature of PCs. Some of us upgrade because we need to (the newer machine is more performant, the old one is hanging by a string) or because we want to switch from a desktop to mobile setup (or vice versa) and some of use upgrade because they want to have the newest product.
And there's no guarantee we'll continue on silicon anyway. Progress will move on.
If we want progress we'll have to move towards something else than silicon. Silicon isn't very well suited for nanotech. Nanotech is still very new to us.