I did it for 8 years. I don't think it's a sustainable way of life, especially if you don't have a partner with a steady and secure income.
Although daily rate is alright, there is no holiday pay so there at least 13 weeks a year where you will earn nothing. No sick pay. No pension. And the occasional (and sometimes not so occasional) dry spells.
It can be quite isolating, as you don't have colleagues of your own but are in a workplace where everyone else does. Some schools will treat you terribly.
And there is very little job satisfaction in it. You don't build relationships with the kids, you don't really make a difference or see the results of your efforts. I would find myself really hoping the phone wouldn't ring because there was just no point to the work I was doing, though obviously I needed the money. I even started developing anxiety around my phone ringing (last minute phonecalls, not knowing where they would send me, not knowing what to expect, or if I would find the school or if there would be parking etc etc etc) and I'm not an anxious person. I don't think - really long term - it is good for mental health, as it is too unpredictable, too isolating and with no sense of accomplishment.
Supply can be a nice breath of fresh air for a short while, or after you retire - but a 40 year career as a supply teacher? You would have no professional progression or pay rise and at the end of it you wouldn't even have a pension. The lack of a contracted income can also make it hard to get a mortgage - and possibly even to rent. It becomes a very stagnant way of living while at the same being completely unpredictable. Meanwhile everyone else around you has stability, colleagues, career progression, a sense of professional accomplishment and can do things like buy a house or get their bathroom redone because they are not having to save up to get through 6 weeks enforced unemployment (and because the bank is willing to take a chance on them).
Like I said, I worked on supply for 8 years. The first couple of years were fantastic - and much needed after leaving an awful school - but the novelty wore off eventually and I am so much happier in a permanent job (though I have moved into a pastoral role - even after 8 years away, I knew I was never going to be a full time teacher again).