First of all I apologize for the length this post ended up being.
Secondly, I'm not going to make predictions on whether or not nappies will be banned outright, though I think that it is unlikely and if it does happen it will come over a period of a decade or more as reusable products and laundry services are brought in to replace them, the economics of it make it impossible to happen over a shorter time. However if it does happen it will likely only be for disposable products that will end up in landfills as we still need to use something and there would be mass outrage if it encompassed all nappy products.
It is believed to take around 500 years for a disposable nappy to decompose (again this is going off other peoples research and things I have read, so it may or may not be totally accurate.) The paper and natural fibers may decompose quickly but the plastics, synthetic wadding and SAP gel does take a very long time. Sure many companies are working on more environmentally friendly alternatives and biodegradability of nappies has been improving a lot but we are still a ways off from mass use and these alternatives may be more expensive (as it's newer tech) and may not be as effective at the job.
Then there is the methane and other gasses produced by the nappes (and waste matter) as they degrade as well as the environmental cost of factories producing all those nappies and shipping them around that has a big negative impact.
I am in many ways supportive of putting heavier restrictions/taxes on and even banning disposable nappies as a means of encouraging people to switch to reusable products that have a much lower impact on the global environment. If they were to be banned (at least disposables) I would not be too affected myself as I would simply go cloth full time but can understand the difficulty some people would have with that and their reasons for not liking cloth nappies.
Now as a thought experiment and mathematics example, let us take a moment to examine something.
At my local nursery they can take up to 42 babies, 48 toddlers (2-3) and 56 preschool (3-5), at once, they are close to max numbers most of the year and have a big waiting list. The average nursery size is about 30 babies, 38 toddlers and 48 preschool. The babies will all be in nappies full time, about half the toddlers will be in nappies/pullups until nearly 3 and some of the preschoolers may also be wearing.
So taking the average size gives us 30 babies, about 16 toddlers and maybe 8 preschoolers for 54 children in nappies.
They are in a nappy when they arrive and most places will (by safeguarding policy) change that soon after drop off, even if they're still dry, due to not having a guarantee of how long the child has been wearing it.
All those children would be changed again at least once before lunch, early in the afternoon and before dinner. Some of them, especially babies, maybe changed more because of how often they go. This totals around 4 - 6 nappies going to waste per child over a day, from 8 AM to 6 PM.
54 children * 5 (average nappies used) = 270 nappies per day
270 * 5 days a week = 1,350 nappies per week
From just one nursery, this is enough to fill one of the large size commercial waste bins to the point where the lid won't close (also ignoring here the fact that they are put in large plastic yellow bin bags by law as it is biological waste and most nurseries will also put each nappy (at least soiled ones) in a small nappy disposal bag. So the plastic takes longer to decompose).
Even if you were to compress these nappies into a block (like a cardboard compressor or car crusher style) one month worth of nappies would still make about a 2 meter cube (maybe as little as 1.5 meters but more likely bigger). This would also cause it to take longer to decompose due to lack of exposure to the elements inside and being more dense.
Now imagine that over a year, 1,350 * 12 = 16,200 nappies and compressed into blocks each month would be 12 x 2 meter blocks with the dimensions 6 x 4 x 4 meters. From one nursery.
As of may 2022 there are 13,056 early years settings registered in the UK of which 10,715 are day nurseries that take children from infancy, some of these may be smaller than average and some larger so let's stick with the average figures here.
16,200 * 10,715 = 173,583,000 nappies per year going to waste from only day nurseries. even compressing them into blocks would give,
12 (blocks per nursery) * 10,715 = 128,580 2 meter cubes.
The average pro football pitch is about 106 meters x 68 meters, that would be 53 rows x 34 columns of the cubes which is 1802 cubes to fill a pitch with one layer.
128,580 / 1802 = 71.3 layers at roughly 143 meters tall (2 meters tall per layer).
This means that just the day nurseries in the UK produce enough waste nappies in one year that if compressed into sold 2 meter cubed blocks will completely cover a professional sized football pitch with a block that is just over the height of the Great Pyramid of Giza! (130 meters) or one and a half times the statue of liberty (93 meters). This does not take into account the 2 or 3 nappies those children use at home in the week or the 5 to 7 nappies they use each saturday and sunday. Nor does it account for the children who attend some other form of day care or simply do not attend early years care at all and are looked after at home.
It is estimated that families in the UK use 3 billion nappies per year for children. If this is accurate that's 17 times more nappies than the day nursery calculations above, 17 pro football pitch sized boxes 143 meters high, Every year! just for childrens nappies, Now think about how many more that would be if you included adult incontinence and ABDL's nappies.
Even if it only took 10 years not 500 to break down completely that is still a MASSIVE pile of waste!
Also bare in mind this is only for the UK and our population accounts for only 0.87% of the world's population at 68,887,011 people.
Just some food for thought.
! There are probably already prototypes out there . 
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