Yeah, I gave an honest shot at using vertical tabs for a few months because it frankly does seem like a more logical use of screen real estate. Web pages don't tend to take up much horizontal space, so you might as well put a bigger list of tabs there where they can all show more text.
For one thing I could just never get used to my normal tab-switching shortcuts moving me up and down compared to left and right. And all my other apps with tabs still use horizontal tabs, so I couldn't fully switch over to that model in my head. Additionally the URL is still at the top so it was more work to glance back and forth between the left of my browser for the tab and the URL at the top which in my mind are more "closely linked" for that to make sense.
But you also highlighted a good point, the limited space of traditional tabs does keep my organization in check. Once I get around the 20-tab mark and I'm unable to see any text beyond the website's favicon, I start feeling dirty and it gives me some incentive to clean up.
I think vertical tabs has the exact same effect of being artificially space limiting if that's valuable to you, without the amount of visible text changing every single time you open or close a tab.
I tend to sit with 20-40 tabs open, which is in the vicinity of how many a vertical tab list can accommodate comfortably, but I get about 4 letters per tab. If I needed to be able to see the text, I'd have to cap a window out at maybe 8 tabs, which is just unreasonable for some workflows.
For one thing I could just never get used to my normal tab-switching shortcuts moving me up and down compared to left and right. And all my other apps with tabs still use horizontal tabs, so I couldn't fully switch over to that model in my head. Additionally the URL is still at the top so it was more work to glance back and forth between the left of my browser for the tab and the URL at the top which in my mind are more "closely linked" for that to make sense.
But you also highlighted a good point, the limited space of traditional tabs does keep my organization in check. Once I get around the 20-tab mark and I'm unable to see any text beyond the website's favicon, I start feeling dirty and it gives me some incentive to clean up.