You are correct. Havertz is not as good as Pogba defensively, he lacks intensity in his game - and as it is, Pogba and Bruno has some defensive question marks. If Pogba left, I’m not sure we should just try to replicate this issue.
That said, I would still sign him, as he’s brilliant, but I’d expect him to bench Bruno in time, rather than partner him. Still early days, but I still have my doubts as to whether Bruno is as amazing as everyone makes out personally. He’s had a great start, and he was needed for sure - but I always saw him as potentially an upgrade for the Mata, Lingard, Pereira role rather than the replacement for Pogba. I think Havertz would be a more worthy Pogba replacement, and we’d need to upgrade one of Fred or McTominay - as we had been saying all along. Nobody was looking at our midfield before Bruno came and saying ‘we just need a better creative midfielder than Pogba’. We wanted improvement in what goes next to him. Replacing Pogba with Bruno or Havertz doesn’t necessarily take the overall level of our midfield up much, if at all. Replacing Fred or Scott with Saúl (purely for example), would have. As we’d have one of them behind Pogba and Saúl, and as a trio, it would have equal quality in both offensive and defensive phases. As good as Pogba and Bruno may turn out, any trio will be better offensively than it is defensively, just like a Havertz and Bruno partnership would be.
If Pogba goes, and we wanted to use his fee for a midfielder, I’d rather a different profile of midfielder to Bruno personally. I wouldn’t be mad if we brought in Havertz, as I think he could be better than both Bruno and Pogba potentially, but we’d still be not much better off than we are with Fred, Scott and either Pogba/Bruno/Havertz. And if we were to look for a partner for Bruno in midfield, my guess is that it wouldn’t be Havertz, or his profile, if we had a choice on a free market.