My last place, albeit in a different department, had a couple of contractors working for them, also for 10 years. Just mental. And the only reason they were there that long is they'd put themselves in that situation and couldn't get rid of them, as they were the only ones with the knowledge to do that job.
Old post but just flicking through the past couple of pages on this thread. At my previous employer, we had loads of long term contractors. One guy, who I somehow convinced to go perm after taking over the team, had been a contractor there for 18(!) years. Within the department, there were about a dozen that had been there for 6 years plus that gradually got whittled down over time.
I've missed the boat on the gravy train but have just recently started contracting myself. I setup my own LTD (to get the ducks lined up), but in the end took an Inside IR35 contract. It means paying a ton of tax but the rates are still very attractive compared to perm work, especially considering how relatively little responsibility some roles have. As a contractor, assuming I were to work the same number of days in a year I'll probably earn about £50k gross more than I did in a more senior perm role, even accounting for pension and bonus. Obviously there's no sick pay, death in service bla bla so it's a slight gamble on remaining healthy but on average I probably took like 1-2 days off sick a year as a perm, even if I was off ill for two weeks as a contractor you make that money back PDQ.
You've then also got the fact that as a contractor you can keep moving around getting paid an exorbitant day rate despite [potentially] adding little value in the early days, as you are new you haven't got the same amount of baggage built up over the years as a perm. Sure, people expect you to hit the ground running moreso than a perm but over say a 5 year period the amount of time spent 'onboarding' with minimal expectations on your output is probably longer as a contractor. I mean as long as you are not a complete idiot you can probably survive at least two months at most places before you get 'found out', and during that time you've earned the average UK annual salary.
So I guess as a newcomer I can judge this objectively and I think even going inside IR35 contracting seems quite lucrative, just not as ridiculous as outside IR35.