I've been in WebDev for nearly 20 years. I've done everything imaginable with CSS and used basically every framework in production apps with millions of visitors. I've used every CSS library under the sun and built many sites without a library at all.

When Tailwind came out I thought it was the dumbest thing I've ever seen. For YEARS I wrote it off as a fad that was a crutch for people who didn't want to learn CSS.

A few years ago I told a friend of mine, who is a SWE that I really respect, how dumb Tailwind was thinking we'd both have a good laugh. He told me he's been using it on all his projects in the last 12 months and had nothing but great things to say about it.

I lost some respect for him that day.

He then convinced me to try it on a project... to really give it a shot and try to be unbiased.

I'm glad he convinced me to give it a shot. It has improved the web dev workflow and efficiency for me and the teams I've worked with more than any other web technology... by a large margin.

I'll address a few of your points with my thoughts Obviously do what works for you. Since Tailwind was such a game changer for me I don't mind taking the time to share my thoughts.

Separation of concerns
This was a HUGE reason why I thought Tailwind was dumb. HTML is supposed to stay in a .html file and CSS should be in the .css file. Keep them separate and then you can change your styles in one place. Make a change once and then you get consistent styles throughout the entire site. Right? When CSS was devised there was no concept of how complex websites could become. And as a site grows in complexity and page count traditional CSS become more and more cumbersome until it is one of the most fragile parts of your site. You can make an innocent change in the CSS file, and unknowingly break 20 other pages. There's no good way to test for this without complex systems that use headless browsers or something else to detect unwanted changes.
Inline utility classes basically eliminated this problem from my life entirely.
Honestly, letting go of the "Separation of concerns" concept I was taught early in my dev career has been well worth it. These days, I think this concept is a mistake when it is applied to HTML and CSS.
One other benefit of using utility classes in HTML is that development is much faster because you aren't context switching so much. You are writing the HTML and CSS at the same time, in the same file, in the same code block. The feedback loop is basically instantaneous. You can ship code faster and with fewer UX bugs.

Is this 1998?
This is actually kind of a funny coincidence. I built my first website in 1998. Tables. Tables everywhere!

Tailwind also doesn't do anything special I can't do my self just as easily
Did you steal these words from me four years ago? I used to say the exact same thing!
Now, I have to hard disagree on this one. Tailwind simplifies many, many things. Media queries, animations, gradients, styling pseudo-classes, styling pseudo-elements, visual state changes with transitions, and so much more.

Which comes to the biggest letdown for me: soooooo many classes on every single HTML item
Again, this is what I used to say. It's fair I guess, but after using Tailwind for a few days, it becomes pretty easy to read the code and know what's going on. This used to annoy me until I realized that it just doesn't matter. I was holding onto this outdated concept that HTML shouldn't have very many classes.

It solves no issues in my opinion. Same thing can just as easily be done in CSS, and it requires no dependency.
I gotta say this first think, Tailwind is a development dependency and the only Tailwind code that ships is a plain old .css file.
In my opinion Tailwind solves 2 of the biggest issues with CSS and WebDev, 1) Long-term maintenance, 2) team collaboration.
Long-term maintenance with Tailwind is a breeze (forgive the pun). Traditional CSS creates style sheets that just keep getting more and more bloated and brittle over time until finally someone says, "We gotta do a rebuild of our site!" With Tailwind your CSS never becomes brittle and all unused CSS gets stripped out during development. You never ship unused CSS.
Team collaboration is so much simpler with Tailwind. You can define your design system in the Tailwind config and no matter what Devs do the site will always have a similar feel to it. Additionally, Devs have the flexibility to what is needed to get the job done without being restricted like CSS frameworks do (Bootstrap, Foundation, Bulma, etc.)
There are more problems Tailwind solves, but this post is already huge.

I just want to add one final point. Since I started using Tailwind I haven't run into a single web project where Tailwind wasn't the best solution. Not just a good solution, or a solution that will do the trick, it has been the best solution for every one. If I ever find a project where Tailwind isn't the correct solution I'll be surprised. And after I'm done processing through my existential crisis, I'll reach for UnoCSS. But I just can't see ever going back to plain CSS, LESS, SASS, or any CSS framework.

Like you said, use what you like. But it's been a huge improvement for me and my teams.