First off, I have no idea how score in osu!mania is a bad thing when applied to osu! standard. This complaint I get the least, because it changes literally nothing about the core gameplay and is really only a "muh spreadsheets" kind of thing, except replace spreadsheets with numbers. It's the same thing.
The real controversy lies within the slider timing window. Personally, I don't see how this is a big issue, as osu! should've had something like this since the very beginning. As it's advertised, osu! is a rhythm game. Woah. This means that playing the song accurately should reward the player, and with sliders now finally getting timing treatment, players need to actually click to the rhythm of the song as opposed to click whenever. If you don't hit a slider accurately, you deserve less accuracy. Honestly as a rhythm game player fanatic, I'd love to see accuracy showcased more, but because osu! also has aiming and combo as features, I can't take those away. The split right now is good, though a bit more of an increase for accuracy wouldn't hurt at all.
Another thing people need to realize is that this is not the final product. Yes, currently it is implemented in the game as a mod, but this is just to showcase what the future entails and to test the new scoring system for those willing to try it out. When they have constructive criticism against ScoreV2, discussion is raised, and depending on whether other people agree or not, things can be tweaked. No system will ever be perfect, but to get as close to perfection as possible is always the goal, and I believe ScoreV2 honestly is making an attempt at this better than ScoreV1 ever could. Because ScoreV2 isn't complete yet, and can be tweaked (as it has been when large outcries happen such as with Taiko big note hits/mania taking combo more into account were either tweaked or outright removed).