I'm not "your demographic" (I'm married and have 1 kid and 2 stepkids, plus assorted extended family...), but honestly, I think asking the kinds of questions it seems that you are asking is probably a good idea for many of us to consider.
In the past 3 years, both my parents-in-law and one stepparent-in-law have passed away (one survives). My dad is in a nursing home with dementia, while my mom is living (very) independently (my parents are divorced). One of my mom's neighbors has recently passed away, and the surviving spouse is at wits end despite having (a) plenty of money and (b) in theory/rationality, plenty of reason to anticipate the spouse's death.
In watching (and in some cases providing assistance to the individuals involved in) all this, and just in general, I've been really struck by the extent to which people experience physical and cognitive declines as they age. Not everyone, obviously, but (ongoing, or temporary but serious) problems of one sort or another are far more common than we in our younger years suspect or plan for, I think. My dad's dementia is of course one extreme of that and one that gets attention (in reporting on the difficulties the elderly and their families negotiate), but my mom's neighbor, for example, has over the past decade or more turned from valuing (and having) a tidy home into a hoarder, the sort who could be on the TV show. Both the neighbor and one of my 2 sets of parents-in-law really struggled with the process of moving from the homes they had owned for decades into more suitable residences (my in-laws moved from a multiple story house that had no bathroom one could access without climbing steps into a ranch home; my mom's neighbor hasn't yet moved but needs to). My mom, who could run circles around most of us, needs to be able to swim regularly to avoid debilitating back pain.
Personally, I'm watching all this and trying to think ahead. I'm fortunate that I live in an area where I want to stay that has many of the features I think are likely to be important down the road; these include:
- good (well, decent, I'm in the US, and not a major city...) public transportation options, including for the mobility impaired
- low crime rate
- good hospitals and other medical care facilities
- good public resources, e.g. the swimming pool my mom uses is a public pool
You may value different things, of course, but I think it's useful to think about. Spending time with elderly people, and asking them about their experiences, might help you get a more visceral sense of what the tradeoffs may be, and/or what you don't want to do without (of course you may already do this and I don't mean to presume you don't, but I just don't know).
As we make modifications to our house (which I hope to live in forever), I'm eyeing accessibility and safety (e.g. minimizing fall risk). There are a number of resources (books, webpages) on design features that facilitate these kinds of things, and I've tried to educate myself and make informed choices. Of course, if you're not anticipating living in the same place it may not make sense to start planning this now, but it's something to keep in mind.
I haven't gotten to where I've planned (much) ahead for a guardian for me or my financial resources ... I think (?) my legal documents name 2 (one my husband, one a stepkid). But clearly this too merits attention and need not be a family member.