X1 didn’t underperform, they were victims of the Produce rigging scandal and were promptly disbanded 5 months after their debut as a result. If they hadn’t disbanded they likely would’ve achieved similar success, if not more, as WannaOne
Seungwoo and Byungchan (and almost Sejun) joined Produce X 101 in a last ditch attempt at saving Victon from disbandment, hoping to achieve a reversal like Nu’est did during Produce S2. Victon ended up being quite successful as a result and achieved their first win while Seungwoo was participating in X1 activities
Unfortunately, most other post-X1 groups didn’t achieve much individual success despite having X1 members in their groups (same can kinda be said about post-IOI & post-WannaOne groups). Only the Iz*one members’ current groups really seem to have benefitted a lot from the members being on Produce
Overall though, if X1 had been allowed to stay together, they’d likely be one of the top 4th Gen bgs, if not the most popular