Hi croquet sorry I have come late to this post, hopefully its still Ok to post my thoughts, to add to everyone else's experiences. Reading your post it made me think there are so many challenges and limitations of shared care, especially as the child transitions and matures into adulthood. The short answer to your question is that IMHO there isnt any one "right time" to dispense with formal shared care.
There is so much thrown into the mix, like the DSC's maturity level, the willingness for parents to be flexible, whether their relationship is cooperative (or conversely whether the DCs are afraid to upset a parent who they sense holds negative feelings or resistance from a parent who see a change as loss of control, etc etc...), proximity between the two homes, even finances can play a part. It's a complex mix of emotions and practicalities.
Your personal situation highlights there comes a point in time where keeping the status-quo doesn't make sense, the DCs may want a change, or the parents feel it is time. We all know the advantage of parents cooperating, it can help matters because adults are more able to see practicalities than teenagers who cant see past their next party/social event/computer game big generalisation but you get the picture!
For you, the distance between their 2 homes is 'forcing the issue', they are having to make a choice because of where their social life is centred. My thought is, if your DP has a strong relationship with them, then even if they see less of him due to social activities, that bond does not weaken provided the face to face contact can be substituted to fill the gaps in time. Does he worry they may drift apart?
Teenagers still need a solid framework, but they have to realise that with age comes responsibility and they cant just expect you/Their DP to drop everything to accommodate their changing needs ( Im not suggesting they are, just a general comment). The trade-off is where a previously predictable care pattern is changed to a more ad-hoc arrangement, which could impact on you, unless you set some early ground rules, such as giving adequate notice if they want to come home to you/DF i.e. Not just ring up on a Thursday to come over that weekend!
Fwiw, I can't see the day when my DSS will ever alter the 50/50 care arrangement, not any time soon. It will only ever happen when he leaves home. At 16yo, he is a sensible young man so coukd be more independent, but the thought of disrupting the status-quo is just not worth it to him. The breakup of my DH with his ExP was so acrimonious that communication has been zero (literally) for the past 6 years. It was a sledgehammer to crack a nut, but DH's naive dream that negotiation and cooperation could be achieved was ridden rough-shod over The 50/50 "parallel parenting" was the only solution ie no contact, DSS transitioning between homes (6 miles apart) via the school bus was the only viable framework that prevented shenanigans ahem keeps tactful silence The downside was totally inflexible and opaque parenting which now means breaking the cycle will be impossible. DSS compartmentalises his life into weeks with us, weeks with his DM, it was the only way he could cope.
petal your comment resonated so much with me, the care pattern is so firm entrenched, that to disrupt it would probably cause DSS distress, and letting life take its natural course to become the catalyst for change is best for his situation.
Ideally, teenagers should have the ability to change, in the knowledge that both parents are supportive and will help them transition into adulthood. If only we lived in a perfect world...