A high quality mattress and king size bed, quality (and ALWAYS fresh) bed linen, good duvets etc is the best investment you can make no matter what your income bracket is, everyone needs this. Totally agree.
I can sleep on anything and be comfortable. A mattress salesman even told me that someone who has my body type doesn't need to spend big on a mattress if they're comfortable with a cheaper one. But there are things I put good money into that others wouldn't. Bloviating about what "everyone" needs or should do is pointless and shows a lack of understanding that people have different values.