There is no way the transition in the secondary will be seamless after losing 3 multi-year starters to the NFL, including a unique talent like Chido. Even accepting the questionable statement that Laguda can be every bit as good as TT, assuming Laguda will be as good is a long leap on its own. Fisher may be a stud, we don't know. Assuming he will be after one good half (the first half was... not so good) is another big leap. Moeller is solid, but not a star. Exactly the kind of guy who does well when there is a lot of talent around him. That talent is largely gone.
I'm not saying they will be terrible, but there is no way they will be just as good or that the transition will be seamless. Plus, they will probably, almost certainly, have more work to do since the front 7 lost a lot as well and QBs will have more time to make throws.
That said, if you meant "the secondary/defense won't suck" instead of what you said, we agree. There are enough pieces there to be hopeful we are not returning to awful or bad.