It is reasonable to assume that the vast majority of the worlds computers are over-powered for the current set of users sitting in front of them.

I disagree. Most consumer computers are so slow you want to pull your hairs out whilst waiting forever for something to load.

I agree. Developers often assume that everyone has the latest flagship laptop. A large majority, particularly in developing countries, only have access to $200-$400 laptops that struggle at running modern applications.

We either have to focus on making our applications more efficient, as we did 10 or so years ago, or make high performance computers more accessible.

I think for many that is a hard drive thing combined with dozens of tool bars, though. Phones have solid state memory so that shouldn't be a major problem.

The solid state memory in phones is a far cry from anything in your computer though and we regularly see devices with storage that's noticably slower than even mechanical HDDs.

Think about flash storage on phones to be more in class of a fast SD card, not the SATA3 SSD in your laptop.

But isn't that mainly because the tools they use (Office etc.) are bloated and slow as hell?

By the time this is in place it's likely a high end phone will easily be sufficient for average computing. You won't run VMs on it but it should be comparable in power to an iPad Pro.