It's impossible because among other issues, attempts to balance can easily lead to new balance issues popping up. And given that you're pitting a small team of balancers against hundreds of players trying to find ways to break the system, it'll never work, especially without doing constant balance updates. Not creating new Pokemon isn't an option for the franchise, since new pokemon and new forms are what drives merchandise which is what fuels the franchise more than anything else. And a fundemental aspect of Pokemon design is that certain Pokemon are MEANT to be more powerful than others. No matter how many small things you tweak, it's impossible to rid the fundemental advantage things like legendaries and Pseudolegendaries get in terms of BST. Even if we ignore stuff like mythicals, cover legends, gimmick pokemon meant to be weak, and not fully evolved Pokemon you've still got hundreds lefts, and you're never going to find a perfect balance where all the remaining Pokemon are usable competitively. Balancing everyone equally at top level play has never really been done for games with much smaller casts and much less variables in terms of customizing characters than Pokemon, and the amount of options Pokemon has will always make it even harder