you don't say what the bike is to be used for or type of cassette you have been using; 9s or 10s. But arguably if you have not used one of the sprockets then it may as well have been one short of that..?..
So realistically you could use 8s, 9s, 10s with a triple and obtain as many or more useful gears as you have been using, or perhaps go with 2x11.
Personally I would prefer the triple because the double would usually leave me running cross-chained more of the time, and the double-shift is larger with a typical double setup. There is also the matter of years of habituation to a triple setup to contend with.
In terms of shifters you can get 8s,9s, 10s triple STIs and if you want a low-cost, easy cable maintenance option there are models with 'washing line' cable routing. I would imagine that even if these go NLA new, you will be able to buy functional used units on e-bay for at least 15 years or so.
In terms of running costs chains and cassettes for 8s are very inexpensive, 9s less so, and 10s/11s less so again.
There is also a double-whammy in operation, in that 10s and 11s transmission parts are somewhat less durable than 8s/9s ones. So in extremis if they are twice the cost and last half as long (say) then the costs of keeping the bike in chains and cassettes are x4 as much.
It can seem more like a triple-whammy in that the RD pulleys on 10s and 11s have narrower bushings (so wear out more quickly and the system is less tolerant anyway) and the cables are more finicky and need more maintenance with more speeds too.
In fact there is another issue which is that 11s road cassettes use a longer freehub body and this results in more dish and a weaker wheel in 130mm OLN form. Weirdly MTB 11s is different and uses the same 8/9/10s freehub body as always so 135mm OLN wheels running MTB 11s cassettes are just as strong as they always have been, but MTB 11s cassettes all have very large sprockets and don't make sense to use with a triple.
Shimano 'road' 10s (with one exception) uses the same shift ratio as 8s and 9s and the ever-shorter cable pull per click caused so many problems in 10s, in combination with underbar cable routing that shimano used a different shift ratio for 11s road and didn't even bother to try the same shift ratio with MTB 10s or 11s, both of which are different again.
10s Tiagra 4700 is an anomalous system in that it (and it only amongst 10s systems) uses the same shift ratio as 'road' 11s which means that it is slightly less fussy about cables and so forth. It pulls ~2.8mm per click, which happens to be nigh-on identical to shimano 8s. You can get 4700 shifters in 3x10 form.
So there are a few solutions that perhaps you might not have considered. For example
1. You could buy 3x10 tiagra 4700 shifters and (using a 4700 RD or an 11s one) have a 3x10 system. [I would use a 135mm OLN rear hub to keep the wheel dish sensible].
2. If you get cheesed off with the running costs of 3x10 then simply by using an 8s or 9s (or other road 10s) RD you can convert the system to 3x8, because the shifter has the correct cable pull for this to work.
At present for a touring build I am considering a triple with 10s shifters and an RD that is not matched to the shifters. I will modify the RD to make it match the shifters. I am also considering using a 7s freehub body fitted with a shortened 9s cassette (so 8 sprockets instead of 9) because there are no end of 9s cassettes that I like but have no use for the smallest sprocket, which can very simply be omitted if the cassette is fitted to a 7s freehub body. The elimination of the (useless, dish-eating) small sprocket means a big reduction in wheel dish which means that the rear wheel is intrinsically stronger so it can be built lighter for a given service. A 135mm oln rear hub with a 7s freehub body can be spaced so that the wheel has hardly any dish on it at all. Built with a ~485g rim such as wheel is practically as strong as a 130mm 10s hub with a ~600g touring rim.
So anyway you have plenty of choices. Arguably a 3x10 system has little advantage over a 3x9 system; you either get another gear ratio (that by definition and the law of diminishing returns you won't use much even if it is a useful size) or a bigger overlap between the ranges offered by each chainring. Most 10s and 11s cassettes start with an 11T sprocket or a 12T one, which you probably wouldn't use anyway.
So there is more than one way of skinning this particular cat. Second guessing what shimano will do (from the POV of spare parts in the future) is IMHO fairly pointless; they have just announced 12s and that uses all different parts to 11s; in a few year's time 11s could look like just another obsolete gearing system anyway. In the meantime they are still launching new products in apparently 'obsolete' formats such as 8s; for example they recently launched Claris 8s shifters with underbartape cable routing...
cheers
