I�m surprised that it makes a significant difference to them unless they are running a hotel or something
.
I guess as you've still got a rather slow broadband speed you don't yet see the difference.
I wouldn't call my speeds "rather slow". More like the same as your parents had, both when on AAISP FTTC and now on 3G mobile. Except my upstream is now faster.
Just as an example of a difference they have is that when they go away (often they do) they download movies to watch from Netflix onto their devices. This is now much quicker than before and able to download multiple movies at the same time.
Twice as fast in fact, so two times as many movies at once. Not exactly "multiple", which suggests considerably more than two.
I think you just blagged them into it

. It also raises the question of why "often" pay a load to go away somewhere to watch what you could do at home. I can take both my connections with me anyway, as I have one on my phone and one on my MiFi router. Each at around the 65Mbps mark here, and whatever I get elsewhere.
Two unlimited data connections at 65Mbps, total cost £42pm. How much is the 140Mbps G.Fast?
It's like when broadband first came out, people didn't think they would need more than 1 meg, now look at the complaints on forums 
Going from 1Mbps to FTTC speeds is a far bigger step than 65 sync to 140 sync

.
My broadband basic info/help site -
www.robertos.me.uk. Domains, site and mail hosting -
Tsohost.
Connection - Three 4G, tbb tests normally 35-45Mpbs down, 65Mbps off-peak, 9-24 up.
BQM
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If you never think of anything off the wall, you'll never think of anything original.