Same as mechanical.

So you could potentially be replacing di two components on a yearly basis?

Here's my unscientific assessment of quality, weight, and longevity for mechanical components:

• 105: Shifts decently, lasts about 1 year of heavy use (~5,000 miles, starts to show wear), heavy
• Ultegra: Shifts very well, lasts forever, still a bit heavy
  
• Dura Ace: Shifts perfectly, lasts forever, light

I have mechanical Dura Ace on an old road bike with >>30K miles and it is still going strong. I started to bump into the wear limits of new 105 after a few thousand miles (more dropped chains and shifts began to lose cripsness).

The DI2 specific components that will see wear are the shifters and the derailleurs. I'm not sure I've known anyone wear out a shifter - much more likely to get damaged through accident or neglect.

FD has little to wear out, rear you can wear jockey wheels but these are easily / cheaply replaced.

My 2012 ultegra DI2 bike has probably 20k under its wheels - still on original battery, shifters, cables - front junction box has been changed but that's upgrading not because there was anything wrong with the old ones - derailleurs are original with new jockey wheels on the rear. Obviously chain, cassette etc have been replaced but that's the same cost / parts as a mechanical system.

Ultegra and Dura-Ace shifters and derailleurs should last 20,000 miles.

Do you sell SRAM?

The only component I ever needed to replace was a FD that broke on the hinges, but it was still under warranty. I've got an Ultegra Di2 6770 system still going strong after 20K miles! I honestly have had more wear and tear (and breakage) with mechanical systems than Di2.

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Every once in a while.

Cripes, how much riding do you do in a year????

Are you referencing the wear components, such as chain, cassette, cables, chainrings, or the shifters/ derailleurs?

No, I consider things like chains, cassettes, cables, and chainrings as consumables, and people tend to use parts and brands outside of the core groupset. The core stuff to me are shifters, FD, RD. Chainrings and casettes are on the bubble between core groupset and consumables. Cassettes do tend to last forever regardless of groupset. And, chainrings tend to wear the same (at least in my experience) regardless of groupset. And both are largely influenced by how well you maintain the chain.

I'm merely trying to weigh the pros and cons of di2 versus a mechanical build. And figuring in long-term costs as part of that

In a few years, you will want Di3.

Lol

My RD cogs were pretty worn after 4 years. That's it. Well... they still were probably fine, but I replaced anyways.
Other than that, I just did the usual chain and cassette replacements.
The RD and FD shift like they were new. My shifters are in great condition.
Might need a new battery one of these days, but I still seem to get a couple months off of a charge so...

 have about 15,000 miles on a 2013 Giant TCR Advanced with Ultegra di2 10sp. Still on all the original parts.

I wouldn't consider 5k miles in a year to be "heavy use". A cyclist who averages 12 hours a week -- not a huge amount by any stretch, would be around 10k miles per year. At that level I would not expect to replace even 105 components every year.

Heavy use is a very broad range. And yes, I would expect a dedicated cyclist riding 12 hours/week to burn through 105 gear. I was surprised with the wear on my 105 after around 5K miles (~6 hours/week training for HIM). After talking to a few bike shop owners, the collective opinion I found is that 105 is probably good for about 5K miles.

What makes you think its worn? Sloppy shifting?

Try cleaning your drive train more than once a year and stop visiting bike shops run by scam artists.

Not counting cassettes, the only meaningful difference between Shimano's top three groups, both road and mountain, is weight. Dura Ace/XTR is less durable because of the titanium used for cassettes.

Dropped chains, noise, and less responsive shifting. My hunch is that the root causes are wear in the pulleys and wear in the derailleur pivot bearings. Maybe the spring return mechanism loses some of its responsiveness that affects its ability absorb sudden chain slack from a FD shift.

I am a pretty excellent bike mechanic, and no amount of adjusting, cleaning the drivetrain, or new chain fixed it.
You made a few bad assumptions here. I cleaned the drivetrain monthly or more, depending on conditions. The shops certainly were not run by scam artists... I am an expert mechanic and can sniff the bull pretty easily; a number of well-regarded shop owners and local mechanics share this common opinion of 105. If you think weight is the only difference between the three lines of equipment, you probably have not looked at it closely or ever ridden with it. I have owned 105, Ultegra, and Dura Ace.