Especially with the amazing interest rates these days...

At some point though, isn't better to get paid more per hour than working more hours? The average senior developer in the area I live - a major metropolitan city on the east coast that isn't NYC - can easily charge $60-$75 hour as a W2 contractor, probably 30% more as a corp-to-corp contractor. It would be a better long term investment to keep learning and gaining skills that make your hourly rate of your salary higher. You can only work so many hours a day.

A side benefit is that the more marketable skills you have, the easier/faster it is to get another job if you get laid off. My quickest time between looking for a job and getting an offer has been 4 days.

What interest are you referring to? Honestly we haven't had interest rates worth considering for years (at least 8 years since a savings rate above 1%). Now you get a rounding error level of interest on savings such that finding coins on the street probably doubles your effective interest rate on savings.

This isn't true on the span of a single year due to market volatility.