I think P!P/TPCi was much better off having this format be Diamond/Pearl-On instead of BW-On with a Banned/Restricted List as Standard/Modified would be the only format that doesn't have a Banned/Restricted List due to sets getting rotated out. The reason why this new format would have a Banned/Restricted List is due to the supposed power creep of the Diamond/Pearl expansions as well as those of the Platinum and HeartGold/SoulSilver blocks. In retrospect, DP-On as well as MD-On for the most part were much more diverse than BW-On and NXD-On in terms of more playable decks with the exception of SableDonk that triggered the last Mid-Season Rotation in the TCG's Standard/Modified format.
The only cards I think that would get Banned or Restricted in this supposed Diamond/Pearl-On format but instead is BW-On for no apparent reason would be Sableye from Stormfront (J-Wittz has a YouTube video explaining why) and Junk Arm (making Catcher functional again like it did with Reversal) although aside from that we'd get back Roseanne's Research, Pokemon Collector, Claydol GE, Broken-Time Space, Professor Oak's New Theory, Cleffa HGSS, Seeker (fun with Jirachi EX and Super Scoop Up), Shaymin (HGSS Unleashed), and of course Machamp SF to "Take Out" Big Basic EX's which should be easier with the nerf on Catcher as long as it doesn't get OHKO'ed by Mewtwo EX or Mew EX. There is a possibility that Claydol GE might replace Tropical Beach in decks that can't afford them in Stage 2 Setup decks.
What's the point of having the format be BW-On instead of DP-On when all we'd get back are Eels and Benchtini which would hit VirGen hard in Empoleon /Dusknoir? Rayquaza EX becomes playable again with Eels back in the format but it was already playable with Emboar as long as you ran 2-3 Tropical Beach or a 1-0-1 Delphox tech for Mystical Fire which is actually pretty laughable compared to Beach. Instead of adding back 3 sets that recently got rotated in a separate format from Standard/Modified you'd have a much bigger and playable card pool in a DP-On format which would pave the way for a more diverse metagame with possibly a Banned/Restricted list keeping the format in check. P!P/TPCi is just being lazy at this point by taking the easy route with a separate BW-On format instead of DP-On where they could be as responsible for managing it as Wizards of the Coast has with their own formats in Magic: The Gathering.