I know this isn't really your point, but after a few longer swims i've started to view getting tired in a different way.
When i'm fresh, i find the honing of my technique is a relatively academic exercise, "streamline body, keep head down" etc.etc. etc.)
However, when you get tired in specific places, then this is a real red flag, and can help to identify areas you need to focus on. For example, on a recent 5k my shoulders started to ache miserably, so i literally HAD to concentrate on finding a solution in the water there and then. I figured I was kind of hunching my shoulder (to accommodate an elbow lead recovery), and when I started to swing the arm around it really helped.
In that way i find tiredness to be a kick in the pants to say "need to do better, focus on x,y,z". I always give myself a break whenever I get tired, I back off the speed and focus on technique….I don't try to hold speed.
Sucks though. I empathise.