Well, take heart. No one wants to leave their home, but your parents may well change their mind. By the time dementia has set in, the carer may well not be able to do it all and not have much choice if they want to keep their loved one safe, or, more likely, the person with dementia will actually miles prefer being in a home.
Here's my piece - I really, really wouldn't wish being trapped in an old house alone with a grumpy senile man who was bigger than me on my worst enemy.
The big risks with home care when someone is that ill are, frankly, to the carers as much as the patient. There's a reason why care jobs are hard to fill - dementia care is long term and, even paid at private rates of 20 an hour, unquestionably the most unrewarding of types of care. Most home carers end up on strong ADs and many have proper suicidal breakdowns, which in your late 70s is no joke.
Patients themselves usually become horrifyingly nasty, if not dangerous (only about a fifth are dangerous, to be fair). Bear in mind the whole brain is failing, not just memory. Personality, skills, instinct, ability to tell night from day - you name it, it flickers, then goes. Sometimes new 'personality aspects' or psychoses appear instead - frightening and tiring for other people.
Often the patient is larger than the carer, mobile and, of course, universally uninhibited in their physical and verbal response to situations that are always strange and frighten them too. 'Uninhibited' is the one of the medical profession's most notorious euphemisms.
The patient can't be left alone for a moment, in case he/she eats the flowers brought for them, hides faeces in their chair, pushes their hand through a glass mirror to talk to the person on the other side, etc etc.