Creating a legal presumption for shared parenting is much more difficult than making changes so that it is a more readily available option. It seems to me that the current system makes a shared parenting option hard to exercise because of
(a) the 'primary physical custodian' designation [which is not legally required: the law doesn't require that there be a primary physical custodian] and
(b) the child support system being built around that person and
(c) there being no standardized way to factor parenting time deviations (at least according to what most lawyers say, as far as I know), thus a 'battle' to deviate away from the formula as it currently is with the primary physical custodian receiving a windfall, sometimes.
It seems to me that a better system would begin by:
(1) asking what sort of parenting plan or time with the child is sought and presenting some options for parenting plans, ranging from 50-50 with various schedules, to various other options, ending in an 'every other weekend and one night a week' and
(2) some kind of child support 'formula' that addresses these different scenarios.
Obviously the state will allow 50-50 shared parenting, but people have to work hard to make it happen. If it were a more readily available option, that would likely make it more readily exercised.
It sounds like what I am suggesting is that there be no official presumption of any custody arrangement. This would be better, I think, than the current at least de facto presumption that the children be with one parent most of the time.