Some good points, but also as you haven't had a degree, you are eligible for student finance and so can do a degree in something different now.
Someone with an engineering degree, that becomes an engineer, reaches a similar level of seniority to yourself and then decides they don't want to do engineering any more, is in a similar situation albeit I take your point that they would at least be able to get through the initial filters for a generic graduate scheme.
I do think some people overegg the argument "a degree is worthless, you may as well do an apprenticeship or start at the bottom and work your way up". A degree is usually the best way, although if you are unsure on what you want to study or why, then you're best to wait a while and try something different first, rather than start a degree you don't like.
On another point I personally don't understand this mentality of people not wanting to work with somebody who has previously been a senior manager with the Big 4 or something. If I was to have a junior that would be doing a relatively low level position and was interviewing and saw a load of students with limited experience or an ex Big 4 manager then he would be top of my list...when offering the job I would say, look this is what the job is, here's what the pay is, I know this is a step down for you but this is all we can offer at the moment, but come in to my team and see what we do and you can give me your views on how we do things and how we could do them better, we'll make the processes better and then you can go off and get promoted off the back of that. It would be like getting free consultancy all the time.
I suppose if the manager just wants to doss around and is afraid of having someone that will realise he's not up to the job then yes thats why you don't want somebody experienced.