A superb and illuminating disquisition by Errikos! I'm most grateful. I'll add a couple of points, then perhaps we'll leave this complex and difficult topic in - hopefully dignified - peace.
1. Pros sometimes also do pro work as a 'hobby'
William mentioned several cases of great composers having produced superb compositions that were not commissioned, nor originally pitched at potential commissions. I can - sort of - corroborate that this kind of activity is not especially rare, by pointing to examples in my pro discipline of hi tech design engineering that are somewhat analogous. (However, I can neither confirm nor deny that these events actually took place, lolol.)
In a certain very large defence development contract back in the late '70s, it was widespread practice among the pro designers to be doing - while at their usual workplace - what used to be called "foreigners" or "homers", i.e. their own personal projects that had nothing whatever to do with the official project at hand. These homers included some marvellous designs, including, for instance, an astonishingly realistic and highly responsive analogue video simulator for driving a car in a variety of environments. There was even a competition informally set up to design and build the fastest model of a drag racer, the only rule being that all parts used had to be what was found in the lab, nothing from outside. This competition culminated in an exciting and well attended race meeting held in the canteen, where about 30 or so models were run in a drag strip and timed with opto-electronic precision.
All of these homer-designers were pros and doing pro work for the official project; but they nevertheless also found the time and motivation to indulge in what can perhaps be called "hobby" projects. Some of these pros were probably just sharpening their creative skills and ingenuity as designers; others perhaps were exploring new skills and new technologies, maybe to be used professionally in subsequent contracts; and some may have been aiming at starting a new business.
Wholly reprehensible of course, but such was the nature of large defence contract management in those days - based on the "cost-plus" model which meant in practice that pretty much whenever prime contractors asked for more time and money for their ongoing projects, they got it. But although lamentable for the customer (the Government), it could be said that at least the pro discipline as a whole benefitted from these lively albeit strictly illicit activities, in terms of general enhancement and development of the ingenuity, knowledge, skills and experiences of the pros who indulged their hobbies with such wonderful enthusiasm and commitment.
So there it is, yet another anomalous take on who is a pro and who is a hobbyist.
2. Pressure - a double-edged sword.
Let me tell you about the most intense pressure I ever experienced as a pro. I happen to be one of the type who thrives on pressure - so long as it's not nefarious or malign - but I know full well that some others suffer badly from high pressure. As always, it's a matter of horses for courses.
One day, NATO High Command discovered to their horror that there was a very serious and intolerable gap in their Order Of Battle for Anti Submarine Warfare. Long story short, I found myself in a huge, very high pressure fixed-price prime contract development project aimed at plugging that gap ASAP. By this time (in the '80s), the UK Government had become averse to cost-plus project management, and in any case NATO could not afford to be kept waiting any longer than absolutely necessary for this project to reach fruition. For this project, UK Government and my company adopted a very tough, efficient and effective programme management scheme from the US Polaris programme. I immediately recognised much of this management scheme from my studies of ancient Chinese philosophy - it had already been described and proposed just before the unification of China about 22 centuries ago and was adopted by the first Emperor of China. Plus ça change!
Needless to say, absolutely no homers showed up on this project. The ethos was far from being laid back. In fact, to my knowledge two people on project were literally taken away by men in white coats. But even worse, there were several suicides. Questions were raised in UK Parliament, but no satisfactory answers were ever provided. I shudder in recollection. The Programme Review Meetings sometimes became figurative bloodbaths and several managers were destroyed along the way. As a matrix-manager, I covered an awful lot of departments and subcontract companies and I was either in or very close to the so-called "critical path" for much of the time. And that sometimes got very scary indeed. But I delivered my milestones, and so did the great majority of others. The project concluded on time, on budget and on specification, then went swiftly into full production. Thereafter, the company quietly dropped the purest and most ruthlessly brutal aspects of the Polaris management scheme - mental breakdowns and suicides are certainly not tolerable concomitants of any kind of professional work in the UK.
Many of us did stuff for that project we didn't even know we were capable of doing. I have to say, I don't recall ever performing as well as I did during those high-pressure years, including doing creative design work (hands-on management was the rule, not the exception).
My response to those who say they conjure up their own deadlines: - pfft. Real deadline pressure can only come from outside. It's an essentially natural part of being the social, cultural creatures that we are. We either endure the pain and discomfort while taking pride in doing the best we can to serve others, or we cop out and bumble along in a happy and contented world of one. I'm pretty damn sure I know where the best results come from, and it ain't the latter. No, I'm certainly not recommending the full-on Polaris/Emperor of China environment; the optimum - except for dire emergency situations - lies somewhere between the first and second stories here.