Bicycle Mechanics - 9-speed vs 10-speed STI

I'm in the market for a new road/light touring bike (undecided yet as to whether to build up myself or buy complete). Since I've last bought a new bike, it seems everything has switched to 10-speed. I'm skeptical -- 9 speeds is plenty (heck, 7 is ok too), and the chains/cassettes seem to cost twice as much.

Which leads me to two questions: first, does 9-speed have a future? If I build up a bike with NOS 9-speed parts, will I be able to get replacements down the line (especially STI shifters)?

Second: if I buy a new bike and get sick of paying for 10-speed chains, how much work is it to downgrade? I'd obviously have to replace the chain and cassette together, and probably the rear derailleur. What about the FD and shifters?

If you "downgrade" from 10 to 9-speed you will have to change the shifters (or use a Jtek Shiftmate), the chain and the cassette. The rear derailleur will work with either and the front derailleur may or may not work well.

My experience is that 10-speed chains and cassettes last about as well as 9-speed stuff did so there is no big downside to going with 10-speed. However, 9-speed will be around for quite a while either as new or NOS components. It's still the standard for most MTB components so chains, cassettes and rear derailleurs will be available for a long time. Brifters may get hard to find and more expensive.

Since you can still buy 5 and 6 speed stuff, nine speed stuff should be available for many years. On brifters, 7 speed stuff is getting scarce, so eventually, nine speed STI will be hard to find.

You might want to consider barcons anyway on a touring bike. I picked up a nine speed 2000 Trek 520, with bar cons, off C/L in my area at an attractive price.

Brifters may get hard to find and more expensive.

I bought some Ultegra 9 speed brifters in 2008 new in box from an online bike store. They are impossible to find anywhere but Ebay these days (and as far back as a year ago too). In hindsight, I feel like I should have gone 10 speed even though at the time I went 9 speed to be able to use MTB cassettes. At least for me, the ergonomics of the 10 speed stuff is far better (I have another bike with Ultegra 10 speed). Sora 8 speed brifters seem to be a thing of the past so unless Shimano decides that 10 speeds is enough, I imagine that eventually Sora 9 speed will have the same fate.

Yes, brifters do get out of date pretty quickly. But I do not think you can stay ahead of it. It will not be long before 11 speed is the standard, and 10 speed stuff gets phased out. You can still find 9 speed Tiagra, which is pretty decent stuff, along with the 9 speed SORA (not so special).

I have two nine speed Ultegra bikes right now, and a nine speed Dura Ace. I have a set of Tiagra take offs on the shelf.

My experience is that 10-speed chains and cassettes last about as well as 9-speed stuff did so there is no big downside to going with 10-speed.

I'd got the impression that 10-speed stuff wears out faster (which is plausible since the cogs/chains are thinner). Have other people found that they hold up as well as 9-speed?