If a company is making $1M over two years and 5% of their customers use legacy browsers you're missing out a potential $25,000 a year, probably more than you'd charge for "legacy browser support" for the lifetime of that website. This simply isn't your decision to make, and if your tech-illiterate boss found out that you don't want to do it because it's hard then they'd be pretty annoyed.
As I said before, either learn to do your job correctly or find alternative employment.