We're always better defensively with Puyol, his age affects how much he can play not how well. As for the lack of pace, that's not the only thing we need in a defender. Who else would you get, Luiz? I'd rather have a slow and smart defender than a fast and stupid one. Having a "world class" starting defender in the lineup instead of Puyol will change very little, positively or negatively. Trying to buy a world class CB is almost definitely not a good choice right now, absolutely knee-jerk and brainless, and would be of no use whereas a regular rotating player is something that actually provides solidity to our options.
Less exposure in defense means overall, it's not like every time we do end up getting exposed it will be a 3v2 breakaway. The pressing not only stops them from creating chances, but it makes the chances that they do create less open and less deadly. Now if you want to keep nitpicking past that and name random wild scenarios, then I don't know what to tell you. Our fullbacks will be there defending when we get exposed, making the space less open, making the likes of Puyol, Pique, and Mascherano not look like fools. Any chance they get is a regular chance, if they do end up getting an unbelievably easy and open chance then even Thiago Silva wouldn't be able to stop them from scoring. We will concede from counterattacks, period. Pressing and an organized defensive system minimizes that as much as possible, our defenders ARE good enough to handle the rest within reasonable ability, but short of getting rocket boots there is nothing more any CB can do and your demands really seem misguided and unreasonable.