Surely the stories of plumbers making 100k a year have some truth in them?
I know lots of cases of plumbers making £2k a week, or even more. It still happens. However, not many of these made £100k per year. Either there were overheads that weren't getting mentioned, or the work was of relatively short duration. Or it was billy bull-sugar...
Its basic economics. With the exception of certain high level specialities, plumbing is not a rare enough skill to command £400 per day take-home, net of expenses. So if that circumstance temporarily occurs, its not long before the customer finds someone who will do the same job for £390 per day, then £380 and so on until the market finds its natural level. What varies is how quickly that happens, which in turn is a factor of a) how transparent the pricing is and b) how high the barriers to entry are - how difficult is it to enter the market as a supplier in terms of training, equipment, scale benefits etc.
Equally, the higher labour rates, the greater the incentive to invest in labour-saving devices and systems - whether thats power
tools, push-fit pipe or factory pre-plumbed units.
What the internet has revolutionised is the transparency of pricing. Free movement of labour and the various time savers have also reduced the barriers to entry, so we are closer to what the economist would call a perfect market, and what the national union of plumbers would call "a race to the bottom".
Under normal conditions, a perfect market works better for buyers than for sellers, so tradesmen may prefer to see some "imperfections" in their own market. However, remember that we are all also buyers, so the processes and mechanisms that have suppressed unusual earning possibilities in plumbing also holds down the price of all those products and services that we want to buy.