A lot of inland California saw a legitimately below average July just last year. And if you think our climate has warmed dramatically, they've pretty much gone twice as far in half the time.
I agree that we could always get dealt a royal flush and see a one-off (or two-off since our cool summers tend to come in bunches) year where everything aligns just right, and in the right months. Though I'd still wager that such a cool summer would be offset by some shoulder season shenanigans relative to our truly cold past eras. And as you said, something early 2010s or early 2000s is the ceiling. Each of which were decidedly less cool in the midsummer than our pre-2000s cool years.
But by the same token, the Central Valley of CA is also quite close to the ocean and is anything but oceanic. Upper level temps stay fixed at a level that enables semi-permanent midsummer heat. And more and more that summer climo with the overwhelming 4CH influence to ward off any Pacific energy seems to be creeping north to us.
Granted, we do have better physical/geographic access to cooler air, not just because of our latitude but because of the larger coastal gaps that exist from Roseburg-north. The Central Valley really only has the Sacramento Delta, and that doesn't even feed directly into the Pacific.
But I think the principle is similar in that Central Valley climate appears to be getting transposed more and more to us and I think that's the crux of our big recent summer shift since we are traditionally a big transition zone summer climate. We're clearly becoming a lot more Mediterranean in the last decade, with fewer and fewer true maritime stretches outside of the heart of our cold season.
Oh and I'm sure it'll be a big shocker, even for the locals. Cali ex-pats will be falling out of recently planted palm trees like lizards in florida after a hard freeze
Then the next year I'm sure nature will compensate with a record warm summer, to "correct" the "deficit" along an accelerating trendline