Also, the problem started only days after i replaced a handle cartridges, although i imagine that what i did would not have affected the hot water output.
I think you said that there are no single-handle faucets, right? So that wouldn't be a factor; it could be if you had a single-handle.
i'm going to flush it out in a few minutes.
So...how did the crud-flushing go? A major concern in doing so is that the crud will come out but will jam open the drain valve. So then you need either to replace the drain valve, if you can, or the unit. As a DIY job, replacing the drain valve is the move. When the licensed plumber is doing it, the return on investment starts to diminish.
Most likely, the issue is crud, but I appreciate that you're the kind of guy that likes to fix stuff and to be sure that the whole thing needs to be replaced before replacing the whole thing. I'm that way, and I wish that more auto mechanics were that way. That's why I like mine; he doesn't replace something until he is highly-confident that that's the cause, and he will look to see if just the bushing/clamp/whatever is available from some aftermarket supplier (Febest is a good one for this), even if the manufacturer only sells the whole unit.
So I'm with you.
And I think the folks here didn't mean to impune you; sometimes we run into folks who want to half-ass stuff to the point that it's either unsafe or just stupid. Doesn't sound ike you're that kind of guy, so we will all proceed from that assumption.
In terms of longevity, you can prolong the water heater life by replacing that anode rod every year or so, if you have enough room vertically above the heater to position one to slide in.
But the biggest issue in terms of replacement is the structural integrity of the vessel. Anyone who has had one give way, as I have, knows that it's not a fun cleanup, and if it isn't in a concrete slab basement with a floor drain, it can be ugly and/or dangerous. If it is, the potential harm of letting it go until it cracks is somewhat diminished, but if it is, in a closet or on the second floor, it's kind of insurance to change it out.
By the way, turning it off and letting it cool so you can do the flushing may well be the thing that causes it to crack after 23 years; that's what happened with mine. I turned the 15-ish-year-old unit off in the morning, and when I came back from dinner, water was running out from under the garage door and a very nice waterfall was coming through the garage ceiling. (It was up in second-floor apartment over the garage. The replacement one now has a protective drain pan underneath it.)
As to the symptoms you have, there will come a point where the crud accumulation makes enough of a difference to be noticeable. As long as one can get through one's shower with no issues, you don't notice whether there was 30 gals available or 20, because it proceeds to reheat when you turn off the water. I don't think the Crud Effect is linear, so there may be a point where it REALLY starts to affect stuff and that threshhold may have been crossed. More likely, though, any individual session of hot water usage by your tenant was probably adequately being served -- until it wasn't. There was 19 gals available and they used 18. But then over time there was now 16 gals available...Then they noticed and called you. So, the "They just noticed it recently" makes sense.
