If you're view these systems as having a particular design, you're missing the point. They're meant to be abstract building blocks that are easy to consistently customize, which makes them a good starting point for your design system/pattern library.

Using the default look is totally cool for prototyping or building things where the visual identity isn't very important, but saying that their design is outdated doesn't make sense. If you want a particular design, you can always go to a marketplace like themeforest and get one that will suit you.

Exactly this. Bootstrap just provides the building blocks and functionality to build websites quicker. Can still style them however you want.

One thing I like about Tailwind is that the autocomplete in Tailwind's VSCode extension really helps to learn the classnames.

I know where you are coming from, but both Bootstrap and Tailwind just have default colors, etc but you are encouraged to customize to your needs.

I was working with something using bootstrap 3 recently...I kinda like the aesthetics of 3 more than 5...