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I like to think that, like myself, sidechains at least occurred to Satoshi early on in his ruminations about scaling modes. Likely we'll never know though. For my part I imagined the name 'child chains', but whatever. Same diff. They are just a logical sub-set of the Bitcoin solution one way or another.
I actually had a similar thought experiment the other day.
Imagine that Satoshi had built sidechains out the box with Bitcoin.
Would we be worse off
or significantly ahead?
I think sidechains are the most natural extension possible to Bitcoin if they can execute what is in the white paper. Of course that remains to be seen but considering just about all of silicon valley invested in their 20 million seed round. My bet is they will.
Satoshi had some ideas about pruning. Apparent that went nowhere, but it seems possible via some hieroglyphics by some of the more clever of the core devs that sidechains might have been evident as to be a better solution some time ago.
Satoshi also had a seemingly weird artifact where even with a small payment, the whole wad went went into the system and change came back out. I remember back when I started paying attention some of the main active people at the time were kind of scratching their heads about what he might have been thinking then basically shrugged and said 'who knows?' The thought occurred to me that between this mechanism and pruning there was potentially quite a bit of room for optimization to deal with some of the scaling issues (but not all.) I actually like the whole idea of sidechains better anyway due to the advantages of flexibility and diversity in addition to their providing a much better scaling potential.