Could hardly agree less.
Players are far MORE flexible positionally than people think. It always makes me laugh when people talk about it as if players were born as a full back or whatever. Often players have actually become professional playing one position but then changed. It's just that sometimes you're more likely to succeed at the top level in one position than another. Jagielka, for example, definitely could play central midfield. He did it a lot for Sheffield United, even in the PL. He would never have got England caps in midfield though, or played so many games for us. He was much better as a central defender but he could play in midfield. Lescott was a centre half who arguably played his best football at full back, Fellaini was bought as a defensive midfielder but many would argue he was better off the striker, Arteta was a central midfielder who became a hero playing out wide, Pienaar very rarely centrally for us but that's where he played at Ajax and for his country. That's just recent Everton players off the top of my head, the world is full of footballers who have changed positions over time.
As I said, it's all about skillsets. Digne couldn't play wide left for us now because that's basically a wide striker, but he could have played there in a Moyes team, easily. His role would have been very similar to what it is from full back now, he'd have excelled less but would have been perfectly comfortable. Delph could play full back because he has a good engine, is positionally disciplined and can tackle. Loads of midfielders could play full back if they needed to, you need many of the same attributes. Football's a fluid game, people don't just specialise in one very specific aspect.