my macs absolutely start to degrade after about 18 months, and I try to keep them limping along for a few more months. I think it’s usually the battery crapping out that causes everything else to sort of overheat and suck

I had my 2012 MacBook Pro for 8 years, before giving it to my dad where it continues to be used (though under far less load than when I had it).

I did get the battery replaced once, it was free. The screen got replaced twice, also for free (the second time they just did it when replacing the battery because the person at the Apple Store noticed a slight wear on the edge anti-reflective coating)

Well, mine admittedly are used primarily for duty so I suppose mine live a rather cushy life - particularly my batteries.

However, I don't think I know how decreased battery life would lead to overheating?

Certainly, dust inside the machine will lead to overheating (or at least, more fan activity) over time. Coats the heat sinks, etc. Perhaps that's it?