If you're just a "cog in the machine" doing research or writing software for a trader, and he ends up sucking, the firm will probably shuffle you around to work under someone else. You have generally useful technical skills, cost less than the trader, and recruiting good technical people is expensive.

If you're a supposedly brilliant trader but cost the firm money taking stupid risk or fail to make any profit to justify your inflated salary and bonus, there's not really much they can do with you.