I'm not sure I'd even call it a mistake. More like an assumption, or maybe a hope, which wasn't particularly unreasonable -- plenty of supporters were touting Talles Magno as a striker before it was tried -- and just didn't work out.
That being said, I do think they stuck with the experiment for far too long after it became obvious it wasn't going to work. But there's an argument to be made it was out of necessity rather than stubbornness.
Anyway, I think what we've done this year is institute a new model of roster development and management. To get there, we had to suffer a bit, especially over the spring and summer. Short term pain, long term gain. But we made it through.
And we damn near made the playoffs anyway. Just think of the matches where we conceded late and either lost or fell to a draw. That's the mark of a young, inexperienced, but talented team.
As a Gooner, I know the story all too well. A couple years ago Arsenal's fan base was calling for Arteta's head. Edu Gaspar's, too. On a scale of one to 10, the frustration level was at eleventy billion. But Mikel and Edu just kept saying, "Trust the process" and went ahead with their business. All of a sudden, it was the next season and lo and behold, the Gunners were at the top of the table and contending for a Premier League title.
That's where we are: At the trust the process stage.