You taken into the chance of the first hand failing (for the mulligan), which is about 1 in 5, and you get approximately 1 in 12 million.
Would people actually be interested in a thread about how probability actually works? I'd be happy to make one if people cared.
Exactly where did this 1/5 estimate come from?
That's the change of having no towers in first hand (and therefore using the mulligan), which can be calculated using hypergeometrics.
Note: The 1 in 5 is for this specific deck, so long as the towers are the only thing that cost 0 in it.
That hypergeometrics is "really hardcore math" for most of people here (me included), but that probability is actually pretty easy to calculate. Basically you just calculate the probability of several events happening one after another:
First the probabilities of single events:
You draw 1st card: 48 cards, 9 pillars, 39 other -> p(1st card is not pillar) = p(1) = (48-9)/48 = 39/48
You draw 2nd card: 47 cards, 9 pillars, 38 other -> p(2) = 38/47
3rd card: 46, 9, 37 -> p(3) = 37/46
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.
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35th card: 14, 9, 5 -> p(35) = 5/14
Then the probability of all those things happening one after another:
p(quite bad luck) = p(1) * p(2) * p(3) * ... * p(35) = 39/48 * 38/47 * 37/46 * ... * 5/14 =~ 0,000000426... =~ 1/2.345.604
p(mulligan) = p(1) * p(2) * p(3) * ... * p(7) = 39/48 * 38/47 * 37/46 * ... * 33/42 =~ 0,209 =~ 1/5