What absolutely kills electrolytic capacitors (such a main PSU caps) is temperature. This is the caps' internal temperature, which depends on ripple-current far more than just the temperature inside the amplifier case. And with large/low-impedance transformers, the ripple current can be 5-6 times the idle DC current drawn by the amplifer: dissipating a watt or so inside reservoir caps is entirely possible, even in low-bias amplifiers. It all adds-up
A typical modern electrolytic cap will have it's lifetime specced by the manufacturer at something like 2000hrs at 105degreesC. On paper just 3months at full load and temperature!
However the actual useful lifespan - usually specced by capacitor manufacturers as 'to the point where rated Equivalent Series Resistance' doubles - essentially follows the Arrhenius equation in being related to running temperature: with lifespan doubling for each 10degreeC drop below rated temp.
So at 40degC that same cap should last 64*2000 or about 14years in continuous use. Try that at 45degC and perhaps one can see why Naim's rule of thumb of replacing main PSU caps at 7-10 years is about right - it really is about the useful lifespan assuming for full ripple current, 24hrs a day, at around 45degC internal temp. This is basic physics - not some Salisbury scam. 7-10 year replacements for a low-bias amp run 24/7 is just prudent.
Stick that same cap inside, say, a Class A amp exposed to heavy ripple (due to the constant high idle bias) and so running at 60degC and it will only last , on average, 3years or so - but you'd never leave such a thing on24/7, maybe 4hrs a day of use and suddenly the caps again might well last 10-12years or more. Equally, a low-bias amp used 4hrs a day might well be fine at 20-30years plus, if the cap seals have held and not lost electrolyte. Use a newer cap rated 3000hrs at 105egC instead and the goal moves back nearly 50% pro-rata; cheap or free when you are replacing the parts anyway.