Any non-trivial piece of code will probably be a pain in the ass to port for a more recent version of the language, and the level of PITA depends on the size of the project X size of the changes in the language.

Case in point, I worked in a project using Ruby. When we migrated from Ruby 2.4.0 to 2.4.6 (yeah, a minor upgrade), it broke spectacularly. Trying multiple Ruby versions, the change was actually introduced in Ruby 2.4.1. After some investigation, a change in Net::HTTP library from stdlib had a change that broke a dependency from a dependency. The fix was just a line of code (we just need to change the adapter used for HTTP communication), however it was two days of work for a minor upgrade.

My current job tried to migrate from Java 8 to Java 11. It also broke multiple services. This one is still in progress, months later.

Python 2 to Python 3 is bigger than both of those version changes (however it is equivalent to Ruby 1.8 to 1.9 changes), so yeah, it does take more time. And like some projects that are forever running Ruby 1.8 or Java 8 (or even worse, Java 6), we will have projects forever running Python 2 too.