It's ridiculous to say Manncehster United made the wrong signing in goalkeeper now that Manchester United have tasted De Gea's ability even more than a decade lol.
This season he has indeed made many blunders but look at what happened last season where he always struggled under the sticks of Manchester United and made some good saves from opponent attacks and even from attacks by his own teammates which were occasionally seen during the Ole Era at that time.
Indeed, this season he has made some fatal mistakes, especially in important matches, but in this case I think saying they (Manchester United) made the wrong signing in goalkeeper is a mistake because it's true that after 12 years De Gea has always been the first choice, it's clear that it's ridiculous to say it's a failure.
After all, David de Gea managed to show the best that is in him, especially during bad times, as you said during Ole's coaching, how he struggled in the defense of Manchester United.