I wouldn't do it.. probably.

But if i (for example) invested 20% of my salary normally, and then one day the market totally crashed, i might consider it if i had the opportunity too borrow all20% of my upcoming yearly salary to invest in a dip instead of trickling that 20% over the year while the recovering market eats my gains.

But yeah trying to game the market typically isn't advised. But also, if you know what you're doing and you can afford the losses, then let people make their own financial decisions.

That really depends on the interest rate, your familiarity with the investments, your debt/asset ratio and your expected ROI.

I would argue every mortgage, real estate loan and small business loan is borrowing money to invest.

Meh, stock market is mostly a casino for the retail investor. If you told the bank you want a loan to pop in the stock market they'd reject you rightly so :-)

Well a lot of professional investing is done on margin. Which is a fancy term for "borrowed money".