In theory, true.
But Gregory is still a complete unknown at this point. It was a no brainer taking him in the 2nd round last year given the value but maybe after seeing him up close, the team realizes that he's nothing more than a situational pass rusher in this league. At that point, I don't think it's wrong to take Bosa at 4 if he's the BPA. If you start drafting because of what you've invested in the psat at a position, that's going to get you in trouble.
No more "trouble" I think than over investing resources that you can't get on the field. If you drafted a running back or quarterback last year, you're not likely to repeat the move, even if a top talent is there in the board. That would be 'best player available' in a vacuum and doesn't make much sense.
Gregory isn't a known commodity and it doesn't matter if he was a 2nd round pick. If Bosa is the best player on the board at 4 (and for all we know, the Cowboys might not like him that much either and it's a moot point) and they take him, I can't argue with them.
I think it "matters" plenty that Gregory was a 2nd round pick, just like it "matters" that Lawrence cost a 2nd and 3rd. If you then need to use #4 overall in yet another defensive end, that's failure. And it's over-investing with draft resources as a result.
I'll argue plenty if the Cowboys draft Bosa at #4, for a number of reasons. Chief among them the fact that they will have then invested entirely too much in terms of draft resources at the defensive end position.