I don’t think you have that math quite right.
The Mets’ liability to Nimmo was $101.25M for 5 years, but they paid $5M of Texas’ share and a $1M bonus to Nimmo for the inconvenience of being traded.
Texas’ liability to Semien was $72M for 3 years.
So, in the end, Texas took on an additional $24.25M. The Mets saved $23.25M.
Of course the Mets lost the trade since Semien has been so bad. The offensive decline was obviously coming although maybe the degree of decline not so clear.
Where the Mets lost out bigtime is the defense. In less than a season, Semien went from being the best in his league at his position to just bad. That is not a normal aging curve.
Semien had a 98 OPS+ last year but 3.2 WAR. This year he’s at 73 OPS+ (obviously terrible) but now the defense is now suddenly very bad and it’s -0.6 WAR.. Obviously there’s still a lot of baseball to be played but for reference, 3.2 WAR would lead all Mets players this year by a comfortable margin.