As @slhck mentioned, you cannot upgrade a device to a new wireless standard. That being said it seems like you are most interested in why 802.11ac Access Points are more expensive than 802.11ac Routers.
802.11ac was approved in January, although we've had drafts since 2011. This is a very new technology and with any new technology you are paying the price for the latest and greatest. To understand the price difference you really have to understand who the target market for each device is, and who would be interested in upgrading to ac right now.
As you correctly identified most SOHO networks will never need an access point, a wireless router is more than sufficient. I would argue that in the average setup (couple of pcs, a printer, some wireless devices) there is little to no need for the increased bandwith that 802.11ac allocates, even a single antenna 150Mbit 802.11N connection will be more than enough. As you stated in your question, in high density areas the jump to 5Ghz will allow a lot less interference as the channels will be less crowded.
Access points are playing a smaller and smaller role in SOHO networks and a larger and larger role in enterprise networks. It is difficult to find an access point that does not come with enterprise level additions. You probably do not need POE for your access point, but most of the access points coming out now are part of a new installation, with a WLAN Controller, multiple access points and POE devices for ease of installation.
Depreciation. Routers sell more than access points. If a distributor has the same amount of 802.11N access points as routers, they will inevitably end up with more APs when 802.11ac came out. This would make the cost of 802.11N AP's go down to try to get rid of them so they can instead stock the new 802.11AC AP's
I think the heart of the misunderstanding is in this statement 'My understanding was anything an Access Point can do, a Router can do as well, and more.' While this is very generally true it is becoming less and less true. We are getting better at building SOHO routers, which means the need for SOHO AP's is diminishing, while the need for enterprise AP's is increasing, this means that the AP's that you are finding have the above mentioned Enterprise upgrades.