As with all things it's a cost benefit analysis. It costs money to support old browsers and a lot of money doesn't come from them do do the math.
I was trying to buy movie tickets online the other day.I tried Firefox then Firefox without ad blockers, then ie, then edge, then chrome on two different desk top machines.
Then my gf bought them on her phone browser.
Thanks mobile first development. How much money must you be costing your adherents?