It may turn out that a home that someone purchases today - new construction or existing - may well serve them at some future date as the home they will remain in as they grow older without thinking about ever replacing it. However, someone can't buy a future condition today. That's the point.
An aging in place or "forever" home is not something that someone typically goes shopping for, notwithstanding those that may find such a hoe because it so clearly meets their needs and those needs likely won't change appreciably over time.
Simply put, aging in place is what happens after many years of living in a home and deciding that it already accommodates a person’s needs well and likely will continue to do so, or that several changes, possibly even some significant ones, can and should be made to the home to create more safety, better access, and more enjoyment - where the decision already has been made to remain living in it.
People make home purchases for a variety of reasons, from pleasure, financial or economic considerations, proximity to activities they enjoy or need to attend (such as employment), they way it is designed. Sometimes, as the days pass, continuing to live in that same home doesn't make as much sense as it did previously. Therefore, that home is not one that seems well-suited for retirement years and beyond.
Of course, modifications are possible and highly recommended, but the basic location, size, style, and appeal of the home needs to be there first before someone decides to undertake the improvements to convert it into a long-term dwelling. Unless people like a home well enough to want to continue living in it - or they procrastinate doing anything about it until several years have passed and they resign themselves to remaining in that home the way it is - they aren't going to devote the energy of planning and executing an improvement program, whether they do some or all of the work themselves or they have someone like us help them.
While it's wise to look for a home that meets someone's current and anticipated needs, it is a real treat for that home to be able to provide for their longer-term needs and remain the only home they will need. They may rightly conclude that they will be quite comfortable living in that home for a long time or that this might be their forever home. They can envision that this could be the case, but it has to evolve over time.