Would XY-on change much anyway? It would rotate very few cards that were truly impactful. I can only think of Eggs, Ace Specs, and Colress outside of Ghetsis.
Thank you for (unintentionally) backing me up, Latte1504. I'm about to explain how and why; please pardon me if it comes across as condescending, but your post that I quoted suggests you aren't seeing the forest for the trees.
Why do sets rotate? The following are steps to understanding, so some are reasons in their own right, but others are not and instead support those reasons where they appear.
1) Business - The powers-that-be don't get a cut from the secondhand market, and I'm not arguing that they should. I'm just pointing out that they want us buying
new cards, to the point that they spend
money and even reward us for playing (Organized Play).
2) Keeping a TCG "healthy" is
difficult. It is easy to mess it up right away, like what actually happened with the Pokémon TCG itself.
3)
The larger the card pool, the more difficult this task becomes. A larger card pool creates more card interactions to track, and please remember that we aren't just checking card-to-card interactions but also card-to-deck, deck-to-deck, etc.
4)
Something similar happens with card design. #3 applies even if you have a static card pool (apart from changes to a Banned List).
However, it gets doubly difficult when you're trying to design and release 500 to 1000 new cards per year. That is because...
5) ...what is bad can become good and what is good can become bad, due to card interactions.
6)
Any banned cards is a sign that the designers "goofed", at least in most TCG's. The larger the list, the more awkward it is to remember and police. Put those together, and companies have an incentive to keep banned cards to a minimum.
7)
Most cards are filler, but even if they are harmless now, remember what I said about future releases making bad cards suddenly good. They can also make good cards even better, mediocre cards good, etc. So,
filler makes balancing the game more difficult.
So,
before we consider that you just named one card at least some people believe is ban worthy (Exeggcute), an
entire class of cards I've said was a mistake and believe are ban worthy (Ace Specs), one useful Supporter (Colress) that specifically rewards big Benches a.k.a. doing what normally helps you win,
and a Supporter that just joined the actual banned list, yeah, I'd say cutting BW-On is a good idea.
Even without those, it would just be a necessary step in "the process".