My rule of the thumb would be - if you can't buy a drop-in replacement and get it delivered in a week, it's outdated.
It doesn't even have to fail in a software/hardware specific way. If your floor collapses or is flooded, and you're using old mainframe with code written in Natural or similar, what's your DR story? Which actually makes me curious - does anyone know what the DR plans look like for companies using 70's mainframes? Let's say a critical S/370 in company basement? (and yes, I'm aware that new z series should virtualise that just fine, but in practice... if you lose a mainframe, what do you even do?)
It doesn't even have to fail in a software/hardware specific way. If your floor collapses or is flooded, and you're using old mainframe with code written in Natural or similar, what's your DR story? Which actually makes me curious - does anyone know what the DR plans look like for companies using 70's mainframes? Let's say a critical S/370 in company basement? (and yes, I'm aware that new z series should virtualise that just fine, but in practice... if you lose a mainframe, what do you even do?)