Point by point:

The interesting point about the brain sensor article wasn't the melting away - it was that there was no immunoresponse.

Right. Like catgut or nylon sutures. There's no immunoresponse there because the irritant goes away. So yeah - if you want to put in an implant that is only going to last a couple weeks, hip hip hooray.

I'm also surprised that you claim "any implant site is subject to opportunistic infection and everything else has a much faster evolutionary cycle.". What about hip implants?

They have a design life of about 20 years. My grandfather went through two. Atrial cardioverters? 10 years. Ventricular cardioverters? 6-8 years. That's the calculus: "Will you be dead by the time this thing fails?" Cochlear implants? Dunno, not my bag... but not as long as you. Breast implants? I've known women who had to get them redone under warranty (typically 10 years); apparently every year, between 1% and 3% of all boob jobs are getting redone.

And these are simple, mechanical things. A saline sack or a titanium jack are doing a lot less than, say, bluetooth.

Saying that we do not have useful implants is just plain wrong, we do, and they are routinely used.

Good thing I didn't say that. What I said was "There are no implants of any kind, nor will there ever be, that will leave you better off than if you didn't have them because your body doesn't like having non-body stuff in it." To be even clearer, I said "Ain't nobody gonna begrudge a blind man an electric eye." And so that we're absolutely lucid, that blind man would much rather have your eyes than a 256-pixel light-dark grid, but he'd rather have the grid than nothing.

They might not make people better at something they did before (except maybe breast implants?), but I doubt it's gonna stay that way for long.

That is because you are uneducated in this subject, and are confusing "wish" with "understand."

In fact, runners with prosthetic legs can already be faster than normal runners (which is just a narrow employ of legs, though, so in my opinion that doesn't count).

Sure - but if I attach Pistorius' springboard to my shoes, I can run faster than Pistorius. Besides which, that's a prosthetic, not an implant. Reductio ad absurdum on that is that a forklift is a prosthetic lifter.

The other idea is, of course, to avoid drawing the immune system's response altogether. And we can already do that, by coating it with biocompatible materials like collagen or PEG.

Wishing doesn't make it so.