The boom had an effect, although the number of IT professionals grew (slowly) after the boom (see page 4 of the link), and I'm fairly sure it doesn't solely explain the collapse in Computing degree applicants.
Also, when looking at average salary consider the continuing growth in the number of people going to university. Also simultaneous collapse in the number of CS grads is likely to help maintain the higher average.
The papers actually quite weak, as there a strong distinction between IT and software development (and CS (G4xx) and IT degree), which the paper doesn't really pick up on. They make the mistake of using HESA data which classifies all IT degrees as computing.
Non-development IT roles in the UK have been dropping dramatically over the last decade due to outsourcing and automation while developer roles have been increasing (roughly 10,000/year).
Salary data in general has a relatively small impact of choice of subject. Most 16-18 years old have a very limited understanding of salaries by degree subject and are often under many false misapprehensions (for example thinking most law students will become lawyers). Choices are far more driven by cultural trends.
I think you're splitting hairs in the first point, it's also strange though that CS grads also have the highest unemployment rate:
http://www.bbc.co.uk/news/10477551. Which seems to contradict your point about developer roles, or speaks to other issues.
However, I absolutely agree that it is cultural trends that drive the choices, which goes back to my original point that now there is a broader cultural understanding of what life is like in the tech sector and that tech hasn't woke up to this yet.
I'm not splitting hairs this is actually a major issue. I run a developer job board, before that I founded a social mobility non-profit that specializes in understanding university admissions and outcomes. I'm intimately familiar with the data.
The BBC article is based upon the JACS classification of Computing which incudes IT subjects. From anecdotal evidence CS has very low unemployment and IT has high unemployment, but due to HESA using the JACS classification a graduate in Business IT is considered the same as a graduate of Computer Science.
If you split universities which only teach CS from those that teach IT, you can see very clearly in the HESA data that Computing has a much higher employment rate at universities which don't teach IT (this isn't clear cut evidence as universities which don't teach IT tend to be academically stronger universities; but it does back up the anecdotal evidence).
I wrote to the universities minister asking him to get HESA to split CS and IT data after he gave a talk at TechHub, but sadly he didn't respond.
Try writing to your own MP. You'll almost certainly get a reply. If he/she's not interested, then try engaging Tom Watson MP's interest. Odd for a ministry department not to respond, though.
Also, when looking at average salary consider the continuing growth in the number of people going to university. Also simultaneous collapse in the number of CS grads is likely to help maintain the higher average.