As ridiculous as it sounds, 50 million for depth is how you compete with City over a 38 game season. Just look at how they have retooled each season - they almost always bring in a new defender and they're never cheap. To me, we have several tiers of defenders in our squad and thankfully they are all of a high enough caliber to have these debates/discussions.
We have one world class defender who has the potential to be the best in his position in the world and that is William Saliba. We have two defenders that are at the very least top 3 defenders in their positions in the EPL in Ben White and Gabriel. We also have versatile defenders who are top 4 quality starters in Tomiyasu and likely Timber. Kiwior and Zinchenko are also useful pieces, especially Kiwior given his versatility.
if you can upgrade one of Kiwior and Zinchenko and bring in a defender in one of those higher tiers, that's how you continue to improve the overall squad quality and quality of depth we have. Personally, I feel like if we brought in Calafiori, he would start at LB and we would have a White - Saliba - Gabriel - Calafiori backline which would be the best in the league IMO. Add to that that you would also have Timber and Tomiyasu as rotation across almost all 4 positions with their versatility and you've got a group of 6 defenders that could make an argument they could start at just about any team in the league. With over 60 games at the least next year, at least 5 of those defenders would get big minutes and we could avoid running them as far into the red as we did this year (Saliba leading the league in minutes and White having to play the entire second half of the season on a hurt knee). I love this move as long as we are also going to strengthen in the midfield and attack.