I don't think it's necessarily selfish to want to say no. I do think it's sometimes impractical though.
My DM developed Alzheimer's about 20 years after my DF died. We'd moved her down to live near us (otherwise a 100 mile one way trip) when she was well and she had a good 2-3 years of fairly good health. Then she had a heart attack. We (my DSis's and I) got her over that - daily small walks for 6 weeks anybody??
Then she was diagnosed with Alzheimer's. At this point she had been type 2 diabetic for 30 years controlled by tablets. Well that went wrong pretty fast. We set up a posset (?) box for her. She forgot to take the tablets. We set an alarm then would get a call to ask why the alarm had gone off. We put a note by the alarm telling her about the box. She'd ring to ask what box! We'd call her to take her tablets and she'd forget by the time she'd got to the box or she'd take two! Gradually little things would drop from her knowledge. We got a district nurse via the GP surgery to giver her insulin injections as that was (rightly) deemed to be the safest method of controlling her diabetes.
We got in a cleaner as she forgot to clean and we'd spend most of any visit cleaning the loo or wiping down the kitchen.
She moved into an extra care housing from sheltered housing. All fine for a while, a minimum care team on call 24 hours a day. They'd check in on her and help with personal care every day eventually.
Then she got muddled with her meal times. Her carer came in at 10am ish one day to giver her some antibiotics the GP had prescribed her to find her cooking her evening meal - she thought she was very late eating dinner. She'd forgotten that she'd not long had breakfast that the care team had made for her! That's pretty normal for people with dementia.
Getting her a taxi wouldn't work. She'd not remember she had an appointment, if we called her to remind her, she'd say she would get ready then forget we'd called! I had been at her door having called 15 minutes earlier to say I was on my way up. She'd say she was getting her coat on the phone but I'd find her watching the TV 'hello love, what are you doing here?' type of conversation! A cab driver would just leave. Someone had to be with her for appointments, she'd forget why she was there or she'd forget what the GP etc had told her. No point them writing it down - without someone with her she'd put it down somewhere and we'd have to call the GP to find out what had been said!
The problem is that these sort of things creep up and there's no real way of planning for them. My MIL used to wander with her Alzheimer's - my mum almost did just once. My MIL nearly set fire to her house by microwaving something for the amount of time it said to oven cook it (so 35 minutes microwave instead of the 5 minutes the product recommended). My mum never did that. Every person with dementia is different so planning is very, very hard. You are constantly fire fighting (almost literally in my MIL case!) running to catch up with latest 'drop' in cognitive ability. That means getting a different mix of help which often means getting a new assessment. And, whilst we finally managed to get mum into a good care home, places are few and far between unfortunately.
So, yes, many elderly people can cope with life until the day they die. Others can be told 'get a cab' or 'let's buy in some help'. But some can't manage, their illness is just too debilitating and, unfortunately, you never know which type your parent is going to be. We can all plan for the problems we foresee - lack of mobility, having to give up driving etc but none of us know if we'll suddenly wake up one morning and not recognise the person we've been married to for 50 years and start on that slippery, awful slope. A plan only works if you can remember it...