You're right for "serious" learning. But for most people, the alternative is not learning anything at all! The best technique is the one that'll actually make you play the piano
Bad technique on unserious learning can still lead to serious injuries like RSI.
Proper technique is also important so that playing is actually fun and not painful. Even just a few lessons on how to properly sit, how to avoid tension in your hands and so on can go a long way. There are video lessons that explain that stuff as well but you need to be very disciplined and really repeat these lessons over and over.
You can absolutely self-learn the piano. People that genuinely don't have the money for a teacher shouldn't let that stop them but it absolutely is harder. Set yourself up for success if you can.
There is no such thing as serious/non-serious learning.
I take weekly music lessons and have been doing that for a decade. After every lesson -- since the very first one -- I am amazed by how many simple things that I get wrong, and how many different areas where I could improve. The teacher just sees/hears that immediately, when you have no idea what you did wrong. And the music just sounds different.
Not only that, the teacher discusses the piece with you, tells you the efficient way to practice (a specific piece or specific passage) etc.
You would probably sound ok to a random stranger, but you quickly hit a bottleneck. You spend a lot of time doing incorrect/inefficient things without knowing it.
