It's not so much switching to something qualitatively better than to something with a somewhat different style.
For example, Apache is chock full of modules with a certain richness to them, they do everything and everything you could possibly think of, with a verbose configuration that usually comes filled with "defaults" by the maintainer, and plenty of options to choose between processes and threads; Nginx is "lean and mean", strictly event-based and minimalistic in almost every way, is usually compiled to include just what you need (and still feels un-monolithic despite modules being statically compiled), comes with sensible defaults and is usually bundled with almost no config.
Some rough analogues: Nginx is to Apache what (Varnish, Postfix, Go, Git) is to (Squid, Exim, C++, Subversion).
I can accept Git >> Subversion and Varnish >> Squid but I'll take you on with sticks and pillows over Postfix >> Exim because that's entirely nonsense, sir, and I WILL NOT HAVE IT.
It's not ">" as much as "<->", ie. not a statement about quality. I know a lot of people love Exim.
The reason I included Exim is that it's a huge monolithic chunk (at least as I remember it, it's been a while) with a very long history (and lots of legacy quirks), whereas Postfix is designed from the beginning to be modern and modular.
For example, Apache is chock full of modules with a certain richness to them, they do everything and everything you could possibly think of, with a verbose configuration that usually comes filled with "defaults" by the maintainer, and plenty of options to choose between processes and threads; Nginx is "lean and mean", strictly event-based and minimalistic in almost every way, is usually compiled to include just what you need (and still feels un-monolithic despite modules being statically compiled), comes with sensible defaults and is usually bundled with almost no config.
Some rough analogues: Nginx is to Apache what (Varnish, Postfix, Go, Git) is to (Squid, Exim, C++, Subversion).