The door to door work has not decreased in terms of opportunity. There are still the same number of meetings for field service as their always has been through the week and at the weekend. Territory is still worked in exactly the same way.
Attendance and participation in the door to door ministry is still as much a requirement for MS and elders as it ever was.
Most congregations organise the carts so as not to conflict with field service events, especially at the weekend. I suspect this is an instruction from the org but don't have a reference to hand. So those on a cart on a Saturday may start at 10:30/11:00 for example, and then it be manned until perhaps 12:30 or 13:00.
The Metropolitan Witnessing arrangement, which is basically the same cart work as everyone else but with a fancy badge so only the select few can take part is slightly different in that this work takes place for far longer, all day into the evening typically, but because it's essentially limited to pioneers then it's OK to conflict with congregation FS events.
So standard congregation cart work is not meant to be a substitute for other FS activities. The reality is that there are undoubtedly some who offset door to door work with standing for hours babysitting a trolley.
The other thing to consider is that Witnesses are encouraged to use videos on the ministry now far more. It's back to the days of the phonograph except now the literature side of it is not printed but electronic.
I don't have any insight on the impact on the ministry in terms of how regularly maps are worked but my wife does not seem to think there are less people out and she goes midweek and on a Saturday. She has not done the cart yet.