There are a ton of desktop email clients that have that design: Apple Mail, Windows 10/11 Mail application, Outlook, Mailspring, whatever.
Of client that have the UI/UX of Thunderbird... well only Thunderbird remained. I get that if you use the email sporadically with only one account, you are better with a client like you described, but at that point you can as well use a webmail.
Otherwise if you work with emails, and you manage tens or hundreds of email each day, Thunderbird interface is great, is compact, is essential, is functional. Not pretty, but works well, it's stable, it's reliable.
Thunderbird is a work tool, and a work tool to me doesn't have to be pretty, it has to work.
Of client that have the UI/UX of Thunderbird... well only Thunderbird remained. I get that if you use the email sporadically with only one account, you are better with a client like you described, but at that point you can as well use a webmail.
Otherwise if you work with emails, and you manage tens or hundreds of email each day, Thunderbird interface is great, is compact, is essential, is functional. Not pretty, but works well, it's stable, it's reliable.
Thunderbird is a work tool, and a work tool to me doesn't have to be pretty, it has to work.