"decline" I've noted is they struggle a bit with their massive growth of customer numbers e.g. phone waiting times. Please do enlighten me further ... 
Yes Plusnet's issues with Phone waiting times could well be to do with the surge of customers who signed up, But Plusnet haven't moved in line with their additions as they should've and have left their "previously loyal customers" in the shadows with service performance issues and then a very reduced support presence as none of us can actually get in touch with reasonable ease to discuss our issues, never mind actually getting them resolved.
Now I understand that not all folk will see the issues but I asked a question recently of users on this forum and if they where happy or not and the response from that was that most where happy but a lot of the happy customers were concerned, there was a significant amount of people who are "no longer" happy with Plusnet... some have left.. I must also note that most happy customers noted they like the pricing at Plusnet and where very happy to make note that nobody else offers as good a price as Plusnet...
So to me this says that Plusnet is a provider popular amongst those who like to keep their utility costs low and no longer appealing to the advanced users like myself and others..
WBC 4400/800 @ 4.2Km > TP-Link TD-W8968v3
FTTC 66000/19999 @ 450m > HG612 > Asus RT-AC87U BQM