That's the whole idea about Petry and Tatar -- no one is saying they should be given away. However, your position on both of these players was quite categorical:
Petry isn't going anywhere. That entire defense after Weber and Petry is AHL worthy. Bergevin isn't going to trade him. I also can't see us trading Tatar.
If you're saying that Petry "isn't going anywhere", you're making him an untouchable, otherwise, he could be going somewhere under certain conditions.
Maybe you can't see the Habs trading Tatar -- however, the Habs's depth is on the wings, so it's definitely envisageable. It's all a matter of return. And surely, nothing prevents the Habs from concurrently exploring replacements for a Petry.
The premise is to find out what they're worth and take it from there. If someone has a sizeable offer, then it behooves you to weigh all aspects -- especially with the quality of upcoming drafts.
There is no "rush" to trade anyone -- but timing as in anything else is important. I don't think Tatar is a headache in terms of replacement. Petry is more valuable, however, the organization has depth on the right side of the D and someone like Brook is close.