I've been doing this full time for about fifteen years, which is a fairly long time (though admittedly not nearly as much as some others here).

I haven't really stopped loving writing and designing software. I still have fun writing code and coming up with clever optimization tricks. The thing that has become draining is the actual act of "having a job".

Obviously I'm grateful to have an income, and I like my coworkers, but the problem with most jobs is that the part I enjoy like ends up being a relatively small part of my day. When I worked for a BigCo there would be weeks at a time where at least half of my day is eaten by meetings and/or emails, and when you do get to work on something technical it's usually not something that's challenging or interesting. A lot of the work ends up being a bugfix or an incremental feature that really doesn't require a lot of thought.

Even startups aren't immune to this. With startups you have the advantage of not being nearly as siloed, but that comes with the double-edged sword of being stuck working on parts of the company or stack that you don't really care about. I deal with fewer meetings but I spend much more time fighting with Kubernetes YAML configurations which I find unbelievably draining, which I might have been able to avoid if I stayed at BigCo.

From 2016-2018, I worked at a MediumCo, where I was able to primarily focus on designing and writing distributed software. I was able to spend a good chunk of time figuring out how to optimize concurrent software, there weren't that many meetings, and I didn't get sick of it at all. I quit that job because I had a romanticized idea of what life at BigCo would be like; if I had the ability to see the future I would have stayed at MediumCo because I didn't like working at BigCo [1].

Anyway, my point is that given my experience, if you can actually work on the things you love, and not just a bunch of ancillary bullshit, I think it's possible you can continue to enjoy it forever. The problem is that most jobs simply aren't like that.

[1] Usual disclaimer; you might be able to dig through my history and figure out who BigCo and MediumCo are in this, and that's obviously fine, but I politely ask that you don't post the proper nouns here.