Quoting A Beautiful Disaster:" There is a big difference in being aware of your child's needs and encouraging them at their own pace, ... [snip!] ... to use a sippy cup after 1 and lead away from the bottle. Encourage you child to grow, not keep them stuck on 'baby' things. "
A bottle isn't a 'baby' thing. A bottle has an artificial teat in order to replicate a human teat. Humans are born designed to suckle a human teat, a biologically normal child would usually do so until are 3-4 years old (of course some would stop earlier or later, but i'm talking the average). Thus a bottle or paci are used to try and meet this need instead when the mother isn't breastfeeding.
The difficulty occurs mostly because the physical action of sucking a bottle is different to that of suckling from a human nipple. The latter encourages healthy oral development whereas an artificial bottle does the opposite. As i said previously, nature hasn't gotten this memo so the child will still seek out a sucking action as is biologically normal. When children are breastfed and have their comfort needs met they don't often latch on to inanimate objects for comfort. So they wouldn't walk around with a paci in their mouth all day at 3 years old like you often see, they would just nurse at bedtime for example as their need is met. When needs aren't met they can be transferred and dragged on for a long time, hence why lots of kids use bottles and paci's at an older age.
Like many others we never had any need for a sippy cup. It's a monkey maker and not a necessity for a child at all. When a child no longer gets their liquids from a teat they can drink from a normal cup. I would class sippy cups, if anything at all, as something babyish and unnecessary. It doesn't meet the need for suckling, it's bad for the teeth and it isn't a regular cup. Going back to the biological norm, children would get all their liquids from the nipple and then as they begin to suckle for other reasons than basic nutrition they can drink from a cup. Using a sippy doesn't encourage this, it could even be said to be holding them back and it's mixed up with the need to suckle and only because their bottle has been removed before they were ready so they are still looking for something.
A child given a bottle for milk and water and who has the opportunity to drink water from a regular cup too would be likely to progress to only wanting milk from the bottle and prefer having water at meal times etc out of a regular cup. Again people seem to distrust their child's ability to know what they need and think parents must force everything otherwise it will never happen when it's just not the case.