Because I have experience with such connections.I never said you can not achieve 100mbps download speed on a 7200/5400rpm hdd,I said "not possible to fully use such connections" by which I meant "not possible to use such connections for full time/most of the time downloading".
And why do you think its not possible to download all the time? as long as the source has proper seeds or good server uptimes ( like steam/origin) then there shouldn't be any problem.
If you're saying the ISP will throttle speeds then in my case that's not true, I've been on this ISP for 4 months with zero downtimes and consistent speeds throughout a download.
here's an image of the average download speed of the earlier file.
10.9 Mib/s is ~91.4Mbps
*i65.tinypic.com/2610olk.jpg
I have seen 5th gen/6th gen i3 laptops running win 8.1/win 10 crawling with disk usage reaching or crossing 80% while downloading at full 75/100mbps.I have also seen desktops with 7200rpm OS hdd & another 7200rpm secondary storage/download hdd struggling while downloading at full 100mbps while trying to run some other cpu/ram intensive task like multiple browser tabs or playing HD high bit rate videos.
Now you're bringing in variables like cpu, multiple browser tabs, palying hd video etc while downloading, Hardisks are supposed to be working even when they're on 80% load, being at 80% load is not a negative thing, if you want to reduce disk usage then either adjust your workload or upgrade your hardware.
I personally have a 5 year old 3rd gen laptop and a 7 year old hp one running on core 2 duo, both of which don't face issues while downloading and browsing/watching hd viseos at the same time. And on my desktop I'll be gaming, have multiple browser tabs and have downloads in the background without any issues.
Also you may not know it but just because a connection speed is 12.5MB/s does not mean a download is being written at the disk too at same 12.5MB/s,it is much more than that.OS has to calculate various things while downloading as each downloading session consists of hundreds or thousands of packets each of which has to be analysed by OS for things like checksum,acknowledgement etc.Just open task manager in any laptop running win 8.1/10,start downloading at full 100mbps & see disk usage going over 80% in a matter of seconds.
That's why I said Theoretically it should be 12.5MB/s, I know in real world scenarios full 100 mbps is not possible regardless of which ISP you're using (even if you're on Google fiber). Again disk usage over 80% is not a negative thing, if that bothers you then you should upgrade your hardware.
Again,either you don't do too much streaming or do it within same room.
First of all most people don't have AC routers(you see flipkart/amazon & best/top selling ones are usual tplink or dlink 300mbps N routers).
Second,most people who bought their laptops 1-1.5 years ago don't have AC wifi adapter.The cheapest AC wifi usb adapter of Dlink costs around 1300 & again most people buy the usual 300mbps usb wifi N adapter costing 600-700.
Third,HD streaming is not that easy even on 300mbps N wifi because of interference from other 2.4GHz wifi networks.Such high speed connections are found only in urban areas where probability of interference from surrounding 2.4GHz wifi networks is also the highest(all the areas I lived in,this was true).You may be able to stream on same floor but on floors above or below(assuming it is a 2 story house) getting good enough signal for HD wifi streaming is very difficult.Even on same floor the usual cheap routers(costing below 1500) won't be able to handle load of simultaneous HD streaming along with downloading on another system.
Fourth,as for 5GHz wifi,yes it has the bandwidth but again 5GHz has lesser range than 2.4GHz(principles of wave physics).Also to fully utilize 5GHz wifi one need AC wifi in all devices which is not that common as of now(unless all of your wifi devices are bought in last 4-5 months).One also needs a good AC wifi router with good cpu & ram to handle increased bandwidth load which is not found in AC routers costing below 3-4k as of now.
First you're bringing in variables like what people may or may not have ( flipkart/amazon sells more 300mbps N routers because that's enough for most people and most people still get N routers because they are on slower broadband connections), flipkart/amazon also has many AC router sales, I personally have a D-link dir 816 which is one of the cheapest simultaneous dual band AC routers out there and it works flawlessly for my needs.
Second, I agree most people still don't have AC enabled devices, but even for a 100mbps connections an N router is more than sufficient, its pointless in getting an AC router if you don't have the internet speed or hardware to match it. And the best way to increase the range of your wifi is to change the antenna to higher dBi antennas or buy a repeater, I personally changed the antennas on my parents tp-ink router from 3dBi to 8dBi ones, much better than buying AC enabled smartphones/laptops.
Third & fourth, streaming on wifi is entirely dependent on your distance and obstacles in-between the router and your device whether on AC or N, 2.4ghz has more interference but better range and 5ghz has more throughput but less range, most devices released in the last 5 years support 5ghz (my laptop, nexus 4, op2 etc).
and you don't need AC to fully utilize 5ghz wifi, to fully utilize AC you need a capable 1gbps internet connection (which most people don't have), AC does increase the throughput on the 5ghz band but you will get the same throughput on 5ghz N router for most internet connections, dual band routers (2.4ghz & 5ghz) router have been in the market for nearly 10 years, but they were using different wireless standards like a/b/g/n and now ac.