I have heard that issue raised by colleagues during the hiring process. At the time I was suspicious about this kind of generalization, but had not been particularly sure how to evaluate the impact.

Subsequently my experience is that developers who have been in the same job for a very long time, have a higher chance of being totally clueless. And also generally not confident with new ideas, not aware of alternative ways of doing things, not exposed to the culture of the broader profession etc.

I am wary of any dev. You could go through this during the hiring process. Most programmers end up doing some maintenance even in new jobs.

But bottom line I suspect this hiring principal is essentially backwards.

In my experience, if you switch projects regularly you become increasingly dependent on your network of previous bosses and colleagues for future gigs. It's stands to reason that you will want to do the best job you can, produce the best code you can so that you have a better chance that you when you leave, you depart from colleagues that will be eager to work with you again in future.

Getting bored easily and professionalism are not mutually exclusive.

Edit: Grammar

True; but you can recognize maintainable code when working in a larger team and ownership of modules changes a lot, which it should for any healthy project.

There are several universal things you can do to ensure maintainability, which are a no-brainer, but don't happen because of various compromises that have to do with time and money - like code-reviews, having no module with a single owner, automatic tests, up to date documents / stories, etc...

If you don't do any of that, it's a no-brainer again that the project will not be maintainable; but it can still be beaten into shape later if the original devs weren't totally insane :)