I don't see the problem with these changes. If Nana could still act while Popo had someone grabbed, it would lead to straight up unfair kill confirms like Nana charing a Smash attack, Popo throwing into a free Nana Fair spike, Popo backthrow into Nana footstool resets, and some other nonsense I'm forgetting.
Ice Climbers were not designed around having ridiculous grab setups, they were made to be strong when there are two and weak when there are one. Maybe it's because I played a lot of ICs on Brawl's awful wifi, but I spent a lot of time learning how to actually play Ice Climbers rather than learning chain grabbing, so this change is nothing but good for me.
Ice Climbers in Ultimate have a ton of speed, a bunch of power and good mobility, not to mention they still have their desyncs, which is what actually matters. They're good characters and always have been, but the grab shenanigans always overshadowed their actual moveset, now all the people who want to play Ice Grabbers can go pick up a character who's actually balanced around getting grabs (Bowser or DK) and the actual Ice Climber fans can play Ice Climbers without getting hate, it's a win win.