Confusing use of plain copula （だ should be だった?）

Question

I am reading Kanon, I will first give the context:

大体{だいたい｝、いつからこの家｛いえ｝の朝食｛ちょうしょく｝は和風｛わふう｝になったんだ…
[Generally,] Since when did we start eating Japanese food for breakfast...

The main character then narrates:

昨日｛きのう｝までは間違いなく｛まちがいなく｝、トーストにゆで卵｛たまご｝だったはずだ
Until yesterday, I have unmistakable expectation that it was toast and boiled eggs

If the expectation was until yesterday, should it not be 「だったはずだった」? He surely no longer has the expectation since he has just had his expectation contradicted. My belief is either I am confused at what the copulae are referring to, or 「までは」 forces the rest of sentence into the past (so the plain copula is present tense... in the past... (time travel tense trouble))

Answer 1

Does it make more sense to you if I reorganize your translated sentence like this?

I have unmistakable expectation that it was toast and eggs until yesterday.

The way I see it, getting something else today doesn't contradict his strong belief that before today it was always toast and eggs. He still has that belief (not sure if "expectation" is an appropriate word here, I think that's taking things too literally).

Answer 2

「昨日までは間違いなくトーストにゆで卵だったはずだ」 is like 「（私の記憶では）昨日までは絶対にトーストにゆで卵だった」, "I'm sure it was toast and boiled eggs / It must have been toast and boiled eggs until yesterday (as far as I can remember)".

Compare:

「XXだったはずだ」 -- "it must have been XX" "I am sure it was XX"
vs　
「XXのはずだった」 -- "it should have been XX" "it was supposed to be XX"

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By the way, the だいたい means そもそも, "to begin with" or "in the first place", not "generally". See definition #2⃣-2 of 「だいたい」 in デジタル大辞泉: 「もとはと言えば。そもそも。『だいたい言い出したのは君だよ』」

(And, as you might have already noticed, the いつから～～なったんだ here is a rhetorical question (修辞疑問文) showing the speaker's dissatisfaction or irritation, not a genuine question expecting an answer.)

Answer 3

筈{はず}だ is Expressing his Current Expectation

If you remove はずだ this sentence only states a fact, as if the speaker is explaining the situation:

昨日までは間違いなく、トーストにゆで卵だった。

Until yesterday it was always toast and boiled eggs.  

By adding 筈{はず}だ it sounds as if the speaking is explaining his expectation:

昨日までは間違いなく、トーストにゆで卵だったはずだ。

Until yesterday it was always supposed to be toast and boiled eggs. (so what happened?)