I agree, saying that David de Gea failed as goalkeeper for Manchester United, in my opinion, is not quite right, but I prefer to say that in his last season he was not able to perform optimally and made several fatal mistakes which resulted in losses for the team. But overall, as long as he's been in a Manchester United shirt I can say he deserves a lot between the sticks and that's not a mistake.
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.
Everyone does mistakes and De Gea does too but I don't think his contribution towards the club can be easily forgotten. He has pulled off lot of saves and I don't know but I think there will be points when other clubs interested in him but he still choose to play for United saving them lot of costs going out and looking for new goalkeeper and let alone secure it. Yeah he did mistakes but I don't think he deserves the sticks at all despite the contribution he made.
Right now, if you ask me there are only few goalkeepers that have been that committed to a club for such a long time. So in my books, despite the mistakes De Gea made I still consider him as one of the legend of the club.
So, will David de Gea get a contract extension or will he really leave? Considering that David de Gea's contract will also expire at the end of this month, and so of course if David de Gea does not get a new contract then David de Gea will leave on a free transfer. What's more, David de Gea's age is no longer young but indeed, at 32 years old, of course David de Gea can still play at other clubs. But indeed, if Manchester United don't extend his contract then Manchester United will indeed make an overhaul of the squad, and I think that's good too.