What annoyed me in particular with that post was that the entire rationale that preceded it completely contradicted the announcement of a (possible) shield-breaking item. All of the lead-up to that point was a clear explanation of how arms races are bad because effects need to get powercreeped to deal with getting hard-countered, and how Riot doesn't want that, yet right after that: "... but also we have Grievous Wounds and might add a shield-breaking item," despite the fact that both of these effects are clear examples of arms races, with GW having had a major negative impact on the state of healing in the game already (the relative commonness of GW now is one of the reasons certain forms of healing, such as lifesteal, are far too strong on their own, imo).
I also feel that while such an effect might perhaps address the current meta by giving players the option to break a marksman slathered in BT + Locket + Janna/Lulu E's shields, it runs the risk of making balance much more difficult in the long run, as is the case with any other hard-counter mechanic. As you mentioned, fighters and tanks with strong shields, such as Sion or Nautilus, might find themselves at an even greater disadvantage if their survivablity gets broken that way, and might require buffs to their shielding power that risk making them unhealthy when left uncountered. Mages who run Seraph's Embrace, and who are currently fine, might similarly find themselves struggling in a meta with that item present, and might end up getting buffed in ways that might make them unpleasant whenever that anti-shield item falls out of favor. Introducing a mechanic like that in the game just creates ripples across the whole of its design, and adds yet another factor to take into consideration when balance is incredibly difficult and volatile as it is already.