Originally, it was not clear that Bitcoin had no fatal flaws, or that it would even work. It was an experiment. It was not wise to buy bitcoins with money that you could not afford to lose because the whole system could have failed at any time and for some unexpected reason.
The statement of investing what you are willing to lose is beyond the scope of bitcoin. That statement has been in existence years before the creation of bitcoin. So, it is totally wrong to constrain its meaning to bitcoin and its early adoption.
The fact is whether bitcoin is failing today or is lasting forever, you shouldn't invest what you can't afford to lose.
This statement originated from the stock trading, where trading volitile assets can be considered as gambling because you wouldn't predict accurately the next market move.
As long as you are sending fiat to something that isn't stable, that statement is for you. Bitcoin can only grow above that statement if it remains decentralized and secured but not volitile.
Technically, you can avoid investing more than you can lose by keeping part of your fund in stable coins (especially decentralized) and move the rest to bitcoin.
Moreover, no one is willing to lose any money even if it's a cent.
Edit:
...you can avoid investing more than you can lose by keeping part of your fund in stable coins (especially decentralized) and move the rest to bitcoin.
A stablecoin such as UST?
I emphasised on decentralized stable coin. Maybe like DIA.