If the proper safeguards are in place of Course the risk of abuse is small
if you needed a legal document stating at what point the person would wish to die (these things would become commonplace if the option was there) plus the medical judgement then there is not much room for abuse at all.
This is something people would be thinking about in their 50's or earlier rather than 'oh shes unable to use the toilet herself and cries herself to sleep in the dark every night lets explore if we want to end it'
I have seen all these things working in care, people sat in the dark for 4 hours because they can't remember how the light switch works, people crying into the night or phoning the police because they believe someone is 'in the house' with them who eventually die in pain and alone covered in shit.
Those are Mild cases (relatively) of old age making someone very uncomfortable at the end of their lives.
So yes, those huge amount of people dying in pain over a period of years is massive compared to the few cases where someone would actually want to kill a relative for their house! Unless their quality of life was massively reduced it would not get through the medical judgement anyway.
Its a bit like saying that people who need morphine or derivatives shouldn't be allowed it in the home because it might be taken/used by a family member. Well yes that is true but the harm from not giving pain relief to a person in pain far outweighs the POSSIBLE harm of someone misusing that medication.
The fact is, there is harm going on right now, people dying slowly, painfully, some being starved to death because that is the only thing we are 'allowed' to do for them and it is at great cost to their personal dignity, their quality of life and to the public funds.
To not introduce something that would remove that harm because of the potential for other harm (that could be dealt with in legislation) is just cruel and barmy.