I have the JCM800 combo, Tom, and take it from me it isn't unreliable. I think there is a little bit of noise caused by a loose wire inside - I really should sort this out, but the in-built noise reduction facilities of the THD Hotplate kills this noise anyway.
So, yeah, I can whole-heartedly recommend it. The fact is, that whilst the amp is older than a brand-new amp, those Marshalls were quality, and that kind of quality lasts. It isn't going to just break down - if it did, would the JCM800's have featured in the backline of almost every rock guitarist of note in the last 20 years?
Slash, Frusciante, Zakk Wylde, Navarro, Metallica, Billy Corgan bla bla bla
If he still can't see what you mean, relate it to an anology he will be able to relate to - furniture, for instance. I mean, people desire Victorian furniture - partly for it's value, but also because of the construction qualities. I mean, these antique dressing tables are still being used today, they don't just fall apart like some modern-day tables would.