As I see it the proposed feature is very useful to API authors and would complete the complicated story of trailing closures. This simple feature would make trailing closures easier for beginners to grasp and overall more intuitive - IMHO. The Core Team seems to consider trailing closures an important feature as they rushed to push Forward Scanning which also improves intuitiveness.
So is the @core-team willing to propose a counter-proposal? @xwu’ proposal has been well-received by the community, however the Core team seems to prefer attributes to the above. To be fair, in the Principles for Trailing Closure Evolution Proposals, the Core Team outlined some possible solutions to address the issue at hand. There has, though, not been a pitch from the Core Team to outright support one of the aforementioned solutions. All in all, I think some discussion with the Core Team - and especially @Ben_Cohen - who was quite active in the “Principles ...” discussion - is integral to complete the feature-set of Trailing Closures in 5.3 as opposed to waiting and causing further confusion in future language versions.