I've been disappointed by how less than ideal some wading boot laces are. many years ago, i replaced some frayed laces with a premium Danner lace. it worked great. Made my boots better because they cinched-up better. Laces are a highly under-rated component of wading. your boots don't work without them. HOWEVER, of course, the next time I went to Danner laces, they weren't quite the same and so didn't work as well. I foget what I fixed the problem with. It might have been para-cord. I would get Simms replacements as a last-ditch. They will rape you for them ( how the F* do they justify charging so much for wading socks when just about anything from Darn Tough, Smartwool and Fit will work way better?) Plus you HAD simms laces and they didn't hold up. Well, maybe they did, given what you said, but I bet you will be more happy with para-cord.
Syl