Quote from Ghost of Cutten:
Trading part-time is much worse than trading full-time. Even where it is possible to do it profitably, you learn much slower and generally make a lot less money. So whilst it provides an income stream and capital, it also makes it very hard to achieve trading success. And don't forget, the upside of most jobs is limited.
Trading well makes money, hence trading is freedom. Trading is also pleasurable for many traders, whereas most good traders hate normal jobs. Many trading strategies don't require constant focus during market hours e.g. long-term trading, automated trading, investing.
The reason to preserve capital is that without it you can't trade. Since capital = initial account size plus P&L minus expenses, keeping expenses low clearly benefits. Many trading strategies also scale, so the income and wealth gets bigger as you get more capital.
A guy spending 10k a year vs 60k a year will be $150k better off in 3 years, assuming no investment or trading gains on that extra capital. If he can make a decent return, he'll be 200k, 250k, 300k+ better off. That's a huge difference in just 3 years.
Living frugally gives you much better odds of success, and means you are richer in future than if you spend lots early on. You said to only start trading if everything is in your favour - well, low expenses is a huge item in your favour, so why are you recommended putting odds against you by spending lots?
In any case, there is never a perfect time to start. There are always downsides. Those that can do, do; those that can't, talk about it.