Well the answer to the question of whether it is realistic to teach yourself the piano for free, is that it depends. What it actually depends on is how much prior knowledge you have and how much confidence you have.
I think that if you felt really confident, you stand a good chance at not only seeking out what you don’t actually know, but you would also have the confidence to go searching for answers when difficulties or road blocks occurred. Conversely, I think that if you didn’t feel very confident in teaching yourself with all sorts of free resources on the internet, then you would either need a teacher to help you along the way or you pay for an online course, where the content is divided up into different modules, so you could measure your improvement and successes along the way.
I really don’t think that it would be possible for a person with absolutely no prior knowledge of the piano or any other instrument could find all the information they need in order to successfully teach themselves how to play the piano for free. However, I am aware that I could very well be wrong on this one – there might be one or several determined individuals that would love to prove me wrong! I would be very happy to be incorrect and if you are the person that proves me wrong, then please let me know.
If you have learned to play the piano before and feel like you need a huge refresher because you can only remember some of what you have learned many years ago, then I think it is very possible to find enough free material on the internet to fill in the blanks. This is because you don’t fit into the category of ‘you don’t know what you don’t know’ because you would be aware of gaps in your memory or gaps in your knowledge that you would know enough to search for the information you need.
If you have played another instrument well and now want to learn to play the piano as a second instrument, then you would most definitely have enough knowledge to be able to find the necessary information.
As I said earlier, I think the more confidence that you have and the more prior knowledge you have got are two very important contributing factors in determining how well you could learn to play the piano by yourself using free resources on the internet.