Even as someone who uses and is fairly happy with ISO week numbers, I think you're quite right.
The problem isn't the ISO 8601 calendar - smart people have thought about these things, and it's probably the best decision to let 2015-W53 contain days in 2016 (gaining in return a contiguous calendar of weeks each with seven days). But 'week numbers year' is obscure enough to deserve a noticeably different format letter.
The problem isn't the ISO 8601 calendar - smart people have thought about these things, and it's probably the best decision to let 2015-W53 contain days in 2016 (gaining in return a contiguous calendar of weeks each with seven days). But 'week numbers year' is obscure enough to deserve a noticeably different format letter.