You are certainly missing out on some epics. At the stage in the game you are pulling these though, 270+, like me, I'm not looking for a ton more epics, even as a low spender. The interesting thing about percentages is that if you're pulling sacreds, you never reach a point where it's guaranteed.
So 12 sacred shards, representing a year, pulled at 2x boosted rates, gives you a 78% cumulative chance to pull a leggo. That's a 1-((1-0.12)^12). If we move up to 24 sacreds, representing 2 years of rewards after day 270, we still only have a 95% cumulative chance to pull a leggo.
To each their own prefence of course, I just personally prefer the guarantee over the 40% chance (that's with the boost).
I'm at 209, and I dare promise you that I will have far from all the epics I'd want by the time I get to the guaranteed champion. It isn't about volume, it's about specifics. I have more legendaries than I can use at the moment because I don't have the books for all of them. Even so, like I said, at worst it's a 4* chicken or a book, so even in the worst case they are not wasted.
The cumulative chance of opening 12 shards and getting a legendary (or rather the chance of *not* getting a legendary) is not particularly relevant since we are talking separate events. You could get 0, or you could get 12. The expected results out of 12 shards at 12% chance is 1.44. Some will get more, some will get less. You may opt to open shards at 6% chance which would bring the expected total down, but then it's presumably because you're trading it for something you value more, which will no longer be an option.
Plus, that missing shard might end up being the one piece you need to complete a summon rush for a fusion, or to get you an extra legendary book or something, so it's not like its potential content is the only value it has.