Decks that use ice bolts to kill are LOL.
The only deck that uses siphons to kill is what, pestal? Otherwise they are used for CC.
So neither card is being affected much, one deck being hurt only slightly (pestal) is not significant enough.
A cap of 75 means there is no change in the way people play which defeats the purpose of introducing a quanta cap.
A cap of 50 means you may need to rethink your deck a bit if you absolutely require more than 50 quanta at any one time.
So there are 2 main decks that use more than 50 quanta, rol/hope and firestall. There are better FG grinders, and making electrum is easy because of the arena, so the newb factor of rol/hope is an irrelevant point. Like what was mentioned about calculating damage, that only happens against a few fgs anyway, so the NEED to fractal dragons and to play a full hand all at once is completely unnecessary. You can have 2 dragons in play, and play 5 more (unupped obviously, or only 4 upped) for a final blow. Now firestall, the deck everyone complains about. No one likes playing against it in pvp, or in the arena. Aside from the complaints about it, any good firestall uses fahrenheit, and that can easily reduce your opponents hp to 50-60 hp, and what does that mean? With 30 quanta, you only need 4 fire bolts, which might not kill, but then your fahrenheit can deal the final hit that turn.
50 cap is needed, it will change up the way some decks are played, which is good, and the fact that people are QQ'ing so much about how much it will hurt firestall is stupid, those same people will complain if they encounter a firestall deck in the arena.
Conclusion, 75 quanta cap changes nothing, so might as well not even have a cap then. But 50 quanta cap nerfs firestall, BUT NOT EVEN THAT MUCH, and it does desperately need a bit of nerfing anyway.