PPS: I am always open to well-thought-out alternative ideas. If there is a simple, well-thought-out proposal for an adaptive blocksize increase, please point me to it.
Sidechains.
Bitcoin does not know nor care about the 'adaptation' (which can happen on multiple fronts simultaneously) and they can in principle be a near to a perfect proxy for space in the native Bitcoin blockchain desired. And, or course, if one really needs an ultra-high level of security, one can always just use native Bitcoin and pay what this kind of security is worth.