What's the word/phrase/idiom for describing something that was the best solution, product, etc in its time. However, after a while, for example a decade, it's proved that it is not as efficient and working as thought.
The phrase state of the art covers what you describe:
The term "state of the art" refers to the highest level of general development, as of a device, technique, or scientific field achieved at a particular time.
So you could say:
That was state of the art a decade ago, but much better techniques are available today.
If a better solution is now in use, the older one is outmoded and has been superseded, meaning that it's been replaced by something both more recent and (by some measure) superior. The implication is that the old way might well have been the best thing available at the time.
If there is not yet a better solution, but you want to show that the current one has flaws which weren't previously apparent, I'd consider something that stresses its age, for example archaic, hoary or timeworn.
obsolescent "going out of use: becoming obsolete". (Obsolete "Of a type or fashion no longer current; out of date as, an obsolete machine.") both definitions from Webster's New Collegiate
Sentence: "Far from being an early adopter, he is content with obsolescent technology until it becomes too much of a hassle to maintain."
Sentence: "It takes only a few years for a piece of consumer electronics to go from state-of-the-art to ho-hum to obsolescent to obsolete."
Sign seen outside MIT lab, years ago: "If it works, it is obsolescent."