The Japanese models of the Apple Watch (which have extra hardware to support the FeliCa system used by cards like Suica, which are very popular in Japan) do support paying without having to double-click the side button first, but only with Suica cards.
(Edit: I'm going to leave the above as I wrote it but add the correction re hardware here - apparently it's the iPhone 7 that uses different hardware for the Japanese model, while the Watch Series 2 uses the same chip from NXP in all models worldwide, but with FeliCa functionality disabled outside Japan, probably due to regulatory approval reasons)
Apple has an extra feature implemented called "Express Transit Card" to support it: https://support.apple.com/en-au/HT207154
Essentially, the card that you assign as your Express Transit Card in settings will automatically be used to pay when you bring your watch near the reader, without having to activate Apple Pay first by double-clicking. Only Suica cards can be assigned as an Express Transit Card, and because FeliCa is widely supported in Japan, you can also use this feature to pay for other things you'd normally use a Suica card to pay for (usually convenience store purchases, vending machines, etc).
Naturally, the reason this feature can't be used with other credit cards is because of the aforementioned security issues around it (some of which are to do with the banks' requirements), whereas Suica cards are limited in a number of ways that makes this use case practical, like it not being a normal credit card (it's like a transit card or debit card where it must be topped up, although it does support auto-top-up).
And as with Suica cards and other systems, when you add a Suica card to Apple Pay the card is actually "transferred" to the device, meaning the plastic card itself stops working. A Suica card is essentially a virtual card identifier that can be moved between cards and devices and only exists in one place at a time. But the Suica companion app supports generating a new card account on-the-fly, so if you have a physical Suica card you want to keep, you can ask the app to create a new virtual card for Apple Pay use (just like getting a new physical card from a 7-Eleven and then immediately transferring it to a phone, etc).
So yeah, it's actually possible with the Japanese models, but wouldn't be an advisable feature to implement for general credit cards.
Trust Japan to get it right. They know how to do things like that so awesomely.
The card remains "ready" for nearly a minute after the double click, plenty of time it have it "primed" before you reach the gate.
I didn't know this, thanks. That would not be too bad if I have a minute to be ready