This concern should be taken very seriously. It was raised in the most-recent pitch thread, and not addressed, there, and has not been addressed in the proposal, at all.
The proposal does not engage in a discussion of the role of juxtaposition as a tool in future evolution, and does not even mention the possibility.
It is surprising to me that the proposal makes no effort whatsoever to address a fundamental, credible concern. I do not expect the proposal authors to have a crystal ball view into future evolution. I do expect the proposal authors to engage in an analysis of the current role of juxtaposition in the language's grammar toolbox and of other roles it might play in the future. At a bare minimum, any proposal that has received this level of concern at the pitch stage should explicitly describe the concern within the body of the proposal and explain why the concern is not valid.
On that basis, alone, this proposal should be rejected. Swift is an immensely valuable project with a bright and open future. A "significant restriction" on that future should be subjected to deeply intense vetting.