Seriously, if it's something you really like doing, then continue on in your boss's memory. After five years you must be very familiar with the content. It would be a shame if you stopped teaching and someone else came along and undid everything you two have built.

the question is:
do you want to continue?
Why would you want to do so?
Is what you are teaching relevant?
Are you doing it for "the money" or for a belief that you have?

Good luck with where you go from here.

But, I don't think one person should have to do the work of two (even if they doubled your pay, which I'm guessing they're not).

I'd now get the full amount, which is largely meaningless in that I have a full-time job and salary.

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They are asked why they don't retire to open up slots for younger professors, their collective responses were golden, and I will paraphrase it in a few sentences.

"As senior faculty, I have the courses already planned, the materials formatted, all I have to do is show up to class and run through the material and call on students. All of the hard work is done, so it affords me income and gives me something to do"

If you have taught this for a few years, you probably have the material down, and just need to fluff it to make it contemporary. Therefore, it's just a matter of sitting in the class and going over the material, possibly changing the balance of teacher-student input to allow more student activity. If needed, grab a senior and have them be a TA.

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I had designed (and led) 7 classes, and my boss had done likewise. I have familiarity with his set, but I'd have to absorb them more than I previously did.