21 year old AV receiver - waiting for the end?

Not worth replacing when it does go, but no reason to junk it until then.  If you are worried that there might be a gap of time between when it goes and when you settle on a replacement, you can probably pick up a used AVR of more recent vintage for next to nothing.  Just verify that it works at time of purchase, keep it around in reserve, and when the Arcam dies use it as a temporary stopgap.  I have an old Pioneer AVR in the basement that I used as a preamp for a few weeks when  my Cary preamp needed some maintenance 

My Harman Kardon Citation 22 amps, Citation 25 preamps, and Citation 23 tuners are over 30 years old. Same with my HK CD players.

Over the past year or two some things started to fail. I'm almost done getting everything refurbished. Some things are getting even better parts than what they came with for better than new sound. This is all costing a fraction of what new equipment would set me back.

I did look at new equipment. For my tastes, it would have cost about 20 grand, not including new speakers. Hey, why not? For a system that sounds good to me it's worth that, but........ How long will the new stuff last? I asked more than a few folks. I said "anywhere near as long as my old stuff lasted?" All I got was laughter and replies of 8-10 years if I am really lucky. No brainer, get the old stuff refurbed.

It sounds great to me as it's what I'm accustomed to. I did buy a bunch of new Audioquest Tower interconnects and it sounds a bit better for it. Way better than the original $2 cords that came with the stuff 30+ years ago.

It's your money and your ears. Do what pleases you. There are places that will repair your gear, you just may have to pack it well and ship it.

I had an old Parasound AV2500 home theater preamp that I loved, so much that when it died (and to answer part of your question, it just died in the night and would no longer turn on), I found a used one to replace it. These go back about 25+ years. But when the second one started acting up (it would switch off for no reason, although disconnecting the power would bring it back) I decided to bite the bullet and get something new. Wow, what a surprise! My new Marantz sounds cleaner, and has so many bells and whistles that the old unit didn’t. I’m sorry I waited so long to take the plunge.