I think it's a good plan.
Where I grew up (abroad), parents are allowed to do this - for children born in the last 4 or so months before the cut off date (and possibly earlier, with special permission/extra needs), they can choose whether to start them in that year or the next one.
It works well - it isn't the case that parents automatically decide that children should defer so that they'd be the oldest. Some children are very ready to start in the original year, and do. But equally, there is no stigma to other ones starting later, for whatever reason. It doesn't mean that there is a new cut-off that makes a different cohort automatically youngest, because there are some who do defer and some who don't, over the span of a few months age range as well. So it all becomes more flexible, and more attuned to what fits for the child.