I think you can look at it from another perspective. These guys are all very good IMO, most of them beyond doubt. The problem is more that they don't trust anyone enough rather than that they trust them too much. Any of them, the old and the new.
For example they complained about Higuain for a whole decade and eventually he left (though he is arguably past it now). He missed a few key goals for sure, but actually with him they got to a World Cup Final and two Copa America Finals. I know that's not enough for Argies, but it's a lot. Same with Di Maria and of course Messi.
Di Maria's case is no different from Alves for Brazil. He keeps being called, no replacement is prepared. When you get in a world cup with Danilo and Fagner, there was an obvious lack of trust to the point William was starting to help them out and that made the team unbalanced as hell. They were all good once, for sure, but what they are today? Di Maria is basically a bench player in a mid league, in 2014 he was a important player for Real. The same goes for Mascherano last cup. He was clearly past it and they missed an oportunity to try to find a replacement for him. They must do now, so they are a tournament behind.
Higuain in other hand, was always bad. This trend started by Guyvarch of the 9 that is useless can be a problem when you need to be decisive at higher level, that is when Higuain failed. They were in those finals despite of him. He was the best only player in the last decade to be able to stop both Ronaldo and Messi from getting more trophies.
I also think the current quality is not as bad as a lot of people are saying. They've had a few golden generations with great players almost in every line, but along with the huge talent most times they have also built successful teams with players like Piojo Lopez, Maxi Rodriguez, Kily Gonzalez, Chelito Delgado, a bunch of mediocre fullbacks, etc etc. Just in the Brazil WC, they had players like Federico Fernandez, Enzo Perez, Demichelis, Rojo, Campagnaro, Lavezzi, Alvarez, Palacio... Not very impressive players along the very good ones. What it does seem to lack now is a couple of tough midfield and CB generals. Mascherano made a huge difference.
But that is a great example, those players had inferior quality, but they are given oportuity to build up a team. In the end, an organized team is better than a mixed team with famous players out of form coming on everytime there is pressure preventing the new players to step up.
They are just coming from scratch every tournament.
The other thing is about the manager. 1978 and 1986 had HUGE managers; these guys still are institutions and not only because they won the WC. Sampaoli is a very particular case. Strong personality but it will only work for him under specific circumstances. I've seen him coach my local team in Peru and he lasted three months and for the only time we almost got relegated that season -- completely lost the players very early on, was not given time, etc. The Argie NT last time was obviously a terrible fit for him, just like post-Bielsa Chile was perfect.
My opinion of Sampaoli is that he is problematic and often has problem to adjust his teams to different circustances, hence why he is not as effective to get trophies as his fame suggests, but it was obvious his style of game would never work without players in top shape and choosen by him. in 2018 he had to bend to the pressure, so the result was awful.