Having seen both of my parents develop dementia at different rates, I'll say that yes it does sneak up. There wasn't a clear day where yesterday they were self-sufficient and now they are not.
It is natural for the sufferer to develop coping mechanisms hide problems from themselves and others. The problems have to grow until the coping mechanisms start to collapse, and then suddenly they are much more evident. Another aspect is the child's denial. It is a significant shift in your worldview to see a parent go from being self-sufficient to dependent. To recognize the myriad day to day functions you took for granted that now require a caregiver's involvement.
Also, the cognitive problems do not develop as a slow and continuous trend. There are different cycles overlaid with seasons, illnesses, and even time of day. So you see fluctuations in their capacity and returning moments of lucidity and function help sustain denial.
I've heard it said, and I agree, the process of caring for someone with dementia includes a grieving process with all the complexities that ever has. You are watching many layers of the person die, sometimes over and over. At times you are faced with the living husk of the person you remember. Or worse, a strange subset of their prior personality traits and memories.
It can take a crisis to force the issue, if families get stuck in denial. Sadly, this also makes later caregiver arrangements more difficult. As far as I can tell, the happiest dementia sufferers seem to be ones who were able to transition into a supportive care environment before they progressed too far, so they had the capacity to adapt and familiarize themselves to new routines. Those routines help them cope in later phases. Those who go down kicking and screaming may be stuck with a care environment that seems (to them) threatening and disorienting.