One of the problems with CS is caused by the huge increase in customer numbers over that period. For instance, I joined in February 2012, and IIRC on
the bottom graph the maximum number connected over a 24 hour period was around 670,000. Now 803,000.
That's ~24% increase and is bound to cause havoc in the support services. Where the additional hardware required is typically on about 90 days lead time, recruitment and training of the CS staff needed is a far more protracted process. In Plusnet's case also requiring additional premises in Leeds. Why Leeds, rather than Sheffield, I don't know, as it wasn't long ago they moved into new offices in Sheffield.
From what I see happening on these forums, (I rarely visit the Community ones), a lot of the poor service of the kind you are receiving arises from getting a less experienced member of staff. If Plusnet had opted to change the first-line support entirely to script-monkeys as with BT and many other ISPs, we would all be far less happy. Unfortunately, inexperienced people are bound to have to follow a few ground rules while they grow into the job. A process that takes years, (not too many if they have brains), rather than months.
I'm not trying to excuse the undoubted fall in professionalism, just explain it. The remedy is in removing any anxiety in relatively new staff of referring upwards things they can't quite cope with. They will inevitably feel this reflects on their competence, and we all know we don't like to give that impression to our line manager.
Removing that anxiety requires an exceptional line manager, who is under the same self-pressure with respect to their own manager. By definition, the "exceptional" are few and far between. It also requires an appropriate number of Level 2 and Level 3 support staff - even more difficult to create.
From the point of view of a customer like you with a non-simple problem the result is frequently unsatisfactory

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