I don't know how many times people need to go over the data with Henderson's run in the team last season but every single performance metric was improved, both defensively and offensively as the team were more comfortable playing high up the pitch, that led to more wins and clean sheets. So while you may think he was made to look like a standard PL keeper, in reality as a team we were much better with him in goal despite his poor game against Liverpool which everyone harks back to as if DDG hasn't had his fair share of nightmares over the past few years. Now, nobody is saying he is the long term answer because simply put, nobody knows and he obviously would need to improve significantly but if we are going to become a team that plays higher up the pitch and dominate games, we certainly are not going to do that with DDG in goal because he can't do it and has actively showed in the past that he is not willing to work on those aspects as his game, for example when the club bizarrely allowed him to bring his own coach in a few years back he reverted to his old Atletico coach which essentially overnight undid any progress that he had been making under Hoek.
You've just quoted the Atalanta goal - goalkeepers ball all day every day btw. And even if it wasn't, it is abysmal, cowardly goalkeeping to just stand on his line rather than going to meet the attacker. In that sense it's not dissimilar to Rodriguez last week, in those situations he effectively shows the striker where to put the ball because he makes himself so small. The direct comparison to instances similar to Rodriguez was the Liverpool 0-5 game where Ronaldo and Bruno both went through in the first half and Allisson was on them like a flash and as such they've barely even been remembered as chances because he did his job simply and gave them nothing to aim at. Best thing about that Atalanta goal was that it horrendous and wasn't even his worst error in that game which says a lot.
Even with that save last night you see the difference between him and a modern goalkeeper. He took a short goalkick which was given back to him, a Brighton striker got to within the same postcode as him so he panicked and just hoofed the ball up the pitch which is where we lost possession, they then knock it around for a period of time prior to playing up the pitch and the goalkeeper is involved 3/4 times in that bit of play moving our attackers around (I'm not saying there weren't instances in that move where we should have won possession because there were), but the basic point is that with a more composed goalkeeper the save most likely doesn't need to be made.
If we floated DDG this summer, who would be interested? Genuinely, in terms of really big clubs, who? Possibly Atletico if Oblak were to move on because they play a deeper line but genuinely, I don't see any other top club wanting him as he is massively detrimental to the overall style of the team in terms of his reluctance to leave his line to either help out his defenders in possession or to come and collect simple balls over the top or crosses into the box. He has no relationship whatsoever with Maguire or Varane on the pitch, you see them look at him and ask where he is all the time, in any given game there are usually two, three, four occasions where him showing for a simple backpass would alleviate pressure and he just doesn't do it & the defenders end up looking like idiots because they've had to put the ball out for a throw in or even lost possession in some instances - Newcastle away being one of the most obvious examples when Maguire was out on the left and looked to play the ball back to DDG with no help and by the time he looked back at the play he was being dispossessed.
In terms of your question regarding do I think we'd concede less with Sanchez or Sa (adding him because I used him as my initial example) in goal? Yes, overall I think we would. DDG has been performing at a career high level in terms of shot-stopping, but despite this has still been badly at fault for a number of goals, particularly through October and November where there were 10-11 goals conceded where you'd expect more from your goalkeeper (two things, I'm not saying that DDG was the only problem on those goals and I'm not saying a different goalkeeper realistically stops all of them either, but certainly most of them IMO) & obviously the two in the last week at Burnley & Southampton which were abysmal. I don't dispute DDG makes stops that they might not at times as well so that is a factor. The most important aspect though that would change, certainly with Sanchez/Sa is that I'm pretty confident that a relationship would build between them and the defence and they are a lot more like what Maguire/Varane are used to playing in front of, and when that trust builds up then the overall defensive unit should improve which is the problem we've seen over DDG's time where he has operated almost as a separate entity to the defence which is not what it's about, it's about playing as a unit.