I don't understand the uproar.
If the V2 Hierarchy are legitimate, then they are perfectly within their rights to lift the obligation here. Is it good or ideal? Certainly not. But can any of you who consider the Novus Ordo hierarchy to be legitimate take it upon yourselves to tell people that they commit mortal sin if they don't attend Mass on Thursday? Who's binding consciences now under pain of sin? Only legitimate Church authority can do that.
Even if you hold the V2 Hierarchy to be "doubtful", then the obligation is doubtful, and doubtful obligations cannot bind under pain of sin. I see the SSPX as being merely consistent.
Please help me to understand how this is incorrect reasoning.