Canon Rumors had a link to an Amateur Photographer interview with some Canon executives at CP+. I thought one of the interesting quotes was this, in response to all of the people asking for IBIS:
"We believe IBIS will work together hand in hand with optical IS lenses, such as the ones you see in front of you, to offer better features."
The wording is interesting. "IBIS will work together... with optical IS..." We all assume that Canon makes a ton of money from optical IS, and adding IBIS could jeopardize that revenue stream. This answer makes me think that Canon's IBIS will only work with lenses that already have optical IS. If your lens has IS, IBIS may add a stop or two, but if it doesn't, IBIS won't function.
Just my speculation. It would give Canon a way to say "We have IBIS!" while still being able to charge a premium for stabilized lenses. Your 50mm f/1.8 STM? Still no stabilization!
The complete interview can be read here
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Mike

Part of why I've been thinking of alternates even for my mild uses and that's sad since I've had Canon since '03.

