The discussion was centred around Martinez and how him being on the shorter side should mean he's utilised at LB with de Ligt and Branthwaite as the CBs. So we're specifically talking about the role of the CB when it comes to aerial ability when defending set pieces. But what those earlier discussions ignored, was the importance of technical ability on the ball for a team that aims to exert itself in possession with the aim of playing the game in the opponent's half. This wasn't the case in the EPL 10 years ago, but it is now with some of the brightest coaching minds taking up roles at a number of EPL clubs with the top of the EPL being dominated by those teams. And aerial ability for a CB with the aforementioned coaching mindset isn't as important because those principles prioritise what you do in possession as a priority and they back that up by defending from the front via the high press in a higher defensive line to keep the opponent pinned in their own half. So in that system, what you do on the ball in the build up phase and off the ball higher up the pitch is more important than winning aerial duels. The more control you exert over the opponent by pinning them in their own half the less box defending you have to do as a team.
City and Liverpool have on numerous occasions scored 90+ goals just in the league by applying the above principles. City and Liverpool have scored over a 100 goals each in the league alone under Pellegrini and Rodgers and their successors (Klopp & Guardiola) have carried on the momentum and consistency by implementing a play style which sacrifices defensive stability for goals.
So with United being a few years away from mounting a realistic title challenge, it makes sense to develop the team by prioritising technical ability first, followed by raising the physical and athletic level of the team, which are two areas that are lacking imo and these areas would give us the biggest improvement as far as playing a more expansive game. I'm not saying aerial ability isn't important but I don't believe it's a good idea to prioritise the aerial threat over ball playing ability and also the ability to cover ground quickly. Now if there's a CB who ticks all the boxes, then I want us to sign that player but if not, I don't think it's a good idea to compromise on ball playing ability to sign a CB who will win more aerial duels. Even Arsenal who you mention as a example, it took them finishing 8th, 8th, 5th and then 2nd before they had put the pieces together . I'm not saying we should ignore anything, and I want us to have a team that can deal with every threat once the team building cycle is complete.
And as far as what Amol is saying, I do agree, and you don't necessarily need your CBs to win the aerial duels if you can make up for their short comings from midfield and fullback. The EPL is more demanding (imo) now physically and athletically compared to how it used to be and the calibre of players being recruited in the EPL is also at a high level. Take Brighton as a example, their midfield with Wieffer and Baleba looks physically, athletically, aerially and technically very good.
Please feel free to disagree with me.