I don’t fully agree with your assessment, but fair enough, it’s subjective and I can agree with you more than I normally do... The point is that I’m not looking at this from a more pragmatic perspective, and therefore looking at it right now, what to do next after the Liverpool disaster. I'm not so focused on the first matches of the season, where Varane/Martinez looked good and solid and Martinez/Lindelof had their (1) lost game (plus the Omonia was poor). I’m comparing here and now and even if the sample size is small, I think that lately, Varane/Martinez haven’t been as solid anymore. Liverpool was of course the worst but also not as solid from game to game. This, plus Varane's fitness issues which make him a 1 game a week player makes me believe our best option is to alternate between those two CB pairs. I absolutely don't dislike Maguire, but he doesn't fit at all unfortunately and should mainly stand in for injuries, when Martinez needs to rest, or when we need to strengthen our aerial defence at the end of a game.
The second note. Adding Casemiro to the team has had a huge impact. The CBs no longer need to face forwards at full speed in 1v1 situation all the time. Situations where the CB risked to be dribbled, like Salah or Antonio coming at Martinez in our last two games, those situations were standard before Casemiro joined.
That's fair. Out of the two, Varane looks to be moving more sheepishly recently and doesn't look as physically sharp as at the beginning of the season.
Out of the remaining choices, in games that matter, I'd say Shaw as a CB is probably just ahead of Lindelof for me to be Martinez's partner to rest/replace a non-fully fit Varane. 1) Shaw has more genuine covering pace, is as good aerially (perhaps not in set piece situations as that's more tactical) and arguably better on the ball from that position 2) it would give Malacia more minutes at LB, who imo has been underplayed and gives us more defensive solidarity on that left channel 3) I don't really trust Lindelof to cover the space on the right channel when Dalot of AWB go forwards/tuck inside; he's shown time and time again, his instinct is to retreat to his penalty box and doesn't push out enough.
The only issue with that is Shaw could do with a rest and his last game as a CB was poor. The only thing Lindelof has is being fresh. It's still a leap of faith for me for either/or imo, hence why I question 'Lindelof looks good with Martinez'.
As for Casemiro, of course he's been a massive improvement and undoubtedly has helped our defence. That still doesn't take away the fact Martinez and Varane have been superior in every other way. I can name on one hand the amount of times e.g Martinez has been 'done' on the channels, James against Fulham, Antonio against West Ham and Salah against Liverpool. Yet for everyone one of those incidents he's also dealt with multiple
dozens more with maximum efficiency and zero fuss, whilst making us more solid and retain possession. Maguire and Lindelof have shown numerous times, they would either dive in reckless (former) or not do anything at all (latter), which resulted in conceding possession and territory.