The topic of aging and possible care is a difficult one and can only be partially solved by previous structural conditions. Often, it is not the physical but rather the mental impairments that are the real problem, which can make living alone impossible. Nowadays, there are technically feasible options for almost every living situation to enable a person to live in their apartment, even if there were no prior structural prerequisites taken into account.
Space or room is of course always an advantage, although older people often need and want less space.
Of course, you can also do "everything," and still, with high probability, only a small part of a later, very individual necessity will be covered.
Despite barrier-free accessibility, the elderly person falls and no one notices, for example because the carpet slows down their shoe or the parquet floor is too slippery. They forget to turn off the stove, and it requires an illuminated switch on the countertop to prevent this, or a metal handle made by a locksmith with a very individual measure at or above their bed to pull themselves up (too much space can be disadvantageous here if supportive walls are too far away). I was almost "enthusiastic" about what exists nowadays and what can be implemented in a "quite ordinary" apartment or on the normal bathtub, etc. (mostly paid by the insurance depending on the care level), especially when the elderly person is still mentally alert. The respective impairments are as varied as people are different, which is why standard solutions can often only solve a smaller part of the problem.
The elderly person usually also does not leave their apartment as often anymore, so the stairs can become rather unimportant, and if they do, they are helped two or three times a week. Because once they can no longer do that, they usually cannot do other things either.
My point is solely in the direction that currently used standards in house/apartment construction, such as "barrier-freedom," should be personally and individually assessed. By the way, I see this identically in the area of security technology in the house, kitchen design, etc.
Of course, there are fundamental standards that should be observed, but the lion's share of a functioning life in old age for me or a relative will certainly not be predominantly ensured by such predetermined standards but is more dependent on the individuality of the person affected and the willingness of the helpers.
I have often heard that people feel "safe" when they implement this or that and, in my opinion, rely too much on the effect of it instead of dealing more deeply and thoroughly with the very personal situation and deciding accordingly, even if it may partially deviate from the standard.
The standard per se is not wrong, only the sometimes overestimated trust in it.
Therefore, for example, it would be important to me in your place that the father definitely has a small, shaded balcony/outdoor seating area, can nicely look out of the apartment and see some of life outside, and maybe someday even option for emergency/temporary to have the possibility to be in a room in your apartment and perhaps the daughter then lives upstairs, etc. Just such considerations absolutely and honestly include in your own reflections and find out how you want/could solve such things alongside the sensible technical things that can often be implemented later according to individual needs anyway.