Investing in 3D printing isn't the same idea as FAANG ten years ago (minus FB, I suppose) given that the mega tech corps were mega back then. It was only a question whether they would continue to reinvest well, and they all have. It is probably closer to investing in Amazon in 1998 (at the time, barely more than a online book retailer) or even AAPL in the early 2000s (whose entry in MP3 players looked little more than a desperate attempt to stay relevant). An index fund of, say, 90s era e-retailers may not have even panned out with AMZN in the index simply from the sheer amount of total failures. In reality, such an index fund would probably have closed in 2001-2002 due to lack of funds, because losses would scare everyone away.
Timing is a secondary problem. Making manufacturing more modular and generic would invite a boom and the inventors will do well, but profitability in such methods is hard to crack. Even if it's a matter of if rather than when, when could be 20 years from now. A secondary problem is the fact that a lot of these companies are now not public and held privately through venture cap funds, which have exploded in the last few years. An index fund of social media companies would not benefit from FB's meteoric rise because it stayed private for so long (unless said "index" fund had access to a financing round, could legally participate, and was willing to take a risk on an illiquid position.)
You also can't plug this kind of investment strategy into a FIRE calculator and estimate a potential retirement off it. Ultimately, if your core isn't in VTSAX or similar, you're taking on immeasurable risk in retirement (in the sense that you can't measure the probabilities) and the conclusion usually ends up that you do this with very small percentages of your portfolio. So, if you take 5% of your portfolio and put it in stuff like this, does it really matter too much?
I'm sure ARK products are fine for what they are (and 0.75% isn't extreme for this kind of fund), but in the end they are simply offering a service that has a customer base and performance is somewhat secondary. They will close the products if interest wanes and open new ones. Survivorship bias is a real problem. In bear markets, money flows toward stability and riskier and illiquid issues get hammered. If AUM hit a certain threshold, the funds close and no one hears about them again.