1) There are many non-FTTH connections that reach speeds of 150Mbps or above. 802.11n can deliver this at close range but only AC can maintain this speed across multiple rooms.
That varies largely on your hardware. I can get my wifi even across the street from my home I get through many walls just fine. The currently available 802.11ac routers usually have excellent 802.11n antennas/range.
DOCSIS 3.1 HFC is slated to deliver close to 1Gbps without fibre to the home within 3-4 years, as is G.Fast
802.11ac will be much more mature and this router will have support when that finally happens.
I only pay for 25mbps speeds as I consider the sweet spot. Slow enough I don't need to buy/lease a docsis 3 modem, fast enough I can play games/watch hd content with no lag. Downloading things at 3MiBps is good enough for me(for now.)
2) Internet is not the only use for a wireless network. Many people use it to transfer data across the LAN,including to/from a NAS drive.
There will always be obscure uses of technology. Most people do not transfer files within their LAN very frequently(if at all.) The people that do may not benefit from 802.11ac enough to merit the cost.
I use netdrive on my laptop to allow streaming of content from my pc to my laptop. I experience no lag and would get no benefit from 802.11ac.
3) Smartphones and tablets *will* benefit from 802.11ac. Laptops do not need an external adapter. 50% of the smartphones, laptops, and tablets in my home have 802.11ac built in. Most laptops can easily be upgraded with an internal 802.11ac adapter which will outperform any USB adapter.
Some of the newest devices do support 802.11ac, however I cannot think of a benefit to having 1gbps speeds on a smartphone/tablet. I think even if they have the capability, the interoperability issues and lack of usefulness continue to make 802.11ac of little benefit to smartdevice users. Smartdevice apps are usually designed to be bandwidth conservative anyways, further reducing the usefulness.
As a person that doesn't buy the newest electronics the day they come out, none of my devices have support for 802.11ac.
4) The size of currently available 802.11ac dongles aren't much different to the size of standard 802.11n dongles. You don't get micro ones yet, but micro dongles of any standard suck balls anyway.
For now, for most consumers I firmly believe it is better that they just wait for 802.11ac to mature a little bit more.
I am happy to have my archer c7 v2 running OpenWRT and consider the lack of 802.11ac a non-issue(for now.)