Based on your question, you might dig the book “Boat of a Million Years.” The author put quite a bit of thought into just that.

https://en.m.wikipedia.org/wiki/The_Boat_of_a_Million_Years

On a long enough timeframe, even the strongest-willed will want to die eventually

One of my books features an immortal protagonist and I've as such thought about this quite a bit. More than the answers already provided here, what I found interesting as a writer was the balance I needed to find between making an immortal detached from mortal values while still being engaging to mortal readers.

Said as a pithy question, if you can outlive everyone's decisions and mistakes, what would it take to make you do anything at all?

As OP mentioned, a lot of replies focus on loss, that friends will inevitably die and objects will break........ we already face that reality with regular life! That's hardly a downside of immortality itself.

I'll say no one can truly know. Unless you are yourself immortal

Friends, family, and lovers dying before you.

Either humanity gradually grows to despise you for your ancient morals

or they don't ever meaningfully surpass where we're at today.

LoL investing and compounding! I literally have all the time in the world.

Idk id be super depressed if I was able to experience my family, friends, family's children, and so on die.

Read this on the largest number every used in a mathematical proof.

Then ask yourself, if you think you could handle this number in microseconds let alone an eternity

https://waitbutwhy.com/2014/11/1000000-grahams-number.html

Everyone else in your life that isn't immoral (if you're the only one who is) dies eventually, so every time you make a friend or start a family, you do so knowing that you will have to watch them all die someday.

Does yer dick still work when you’re like 150?