I was a Swan authorized service center for the Northeast from 1968-1974 and worked on lots of 350s, 400s, 500s, etc.
I'd never in a million years recommend replacing anything until I knew they needed replacement. If stuff fails, it failed; but if it didn't there is no reason to replace it, because it may well last another 40 years. There's really no way to know, but most parts don't routinely fail just because they're old. The 500C didn't especially stress anything.
The 6LQ6s can last decades if you know what you're doing, or go bad in one hour if you don't. Mine lasted at least 22 years, and when I sold the rig it still made full output power, which was about 150W (same as it was when new). There's no trick to this other than not operating out of resonance.
Unfortunately, some hams and a lot of CB operators (who got their hands on these rigs) didn't really know what they were doing and went through lots of tubes.
A 6LQ6 can handle about as much continuous power as the revered 6146B can.
Problem is the manufacturers had ambitious marketing departments who decided to rate 200W rigs at 500W and stuff like that.
I'd never assume anything needs servicing until I've proven it needs servicing. Having been a service tech for years while I was still in engineering school, our first rule was "Never fix what's not broken," as there's a 50/50 chance you'll make it worse.
What if soy milk is just regular milk introducing itself in Spanish?