Thank you rusmanarief and Jan Dembowski for your explanation :)
I agree that one should never have to modify the core WordPress files.
Yet, once you have upgraded your site and it no longer loads - what else is there to do? In my case I couldn't access my site nor the admin-panel at all, and the only way to get it to work was by doing just the above.
Again, I agree that it should never be necessary - which is why it might be a good idea to have WordPress disable all plugins directly after an update, and revert to a default/safe theme. Call it a fail-safe upgrade system perhaps, but it would help prevent these issues as some of us have experienced.
Again, my thanks for everyone's help and assistance - I learned something from it all :)