Is it worth holding onto these booster boxes? or the japanese alternatives?
I’ve asked a lot of friends and they have said there’s just too much printing such that buying these boxes is equivalent to sitting on penny stocks.
However, this generation also introduced the white glossy cards HR so I’m not sure.
And some store owners I talked to say that there was an incident with roaring skies reprint even though the Pokemon company said they wouldnt do it
Frankly, it is really up to you to decide whether to purchase booster boxes as an “investment” for the future… There are multiple factors to take into consideration that vary based upon your personal financial situation and intentions - do you have sufficient disposable income for the booster boxes; how long are you willing to sit on the product…
I am probably within the minority right now, but I am a big believer that XY Evolutions has a lot of potential for the future! The vast majority of my collection is WotC-Era Cards; being that Evolutions was a “re-print” of the Base Set got me excited! Additionally, I saw how crazy kids in my neighborhood were when this set was first released, they all LOVED IT! That was actually the biggest reasoning behind my investment into this set. I have purchased multiple cases (English and Japanese); however, the money that I spent on this product did not affect me in such a way where I am strapped for cash and/or missing-out on other purchases for my collection / business. I am planning and willing to sit on this product for YEARS!
That’s really what you have to think about… The current sets are being printed into oblivion - and the print-runs are very long (meaning even more product). Plus, everyone and their mother is into PSA Grading, so the POP Reports for these newer sets are INSANELY HIGH! If you’re hoping to purchase booster boxes and hold onto them for a few weeks, months, years… and turn a decent profit, it’s not going to happen for you. However, if you’re willing to hold product for an extended period of time, it might just pay-off for you in the long-run - nobody can tell for certain what will occur though, so it will always be a gamble with these newer mass-produced sets.
what about sitting on the HR singles?
Imo this is the biggest chance for growth in cards from packs.
These cards suffer a lot from centering problems too!
It seems that the recent trend is that Pokémon prints a very large quantity of products, causing market prices to be quite stable, which means almost no profit for investors and sellers - great for buyers and just pure collectors. I would say you must have great patience and be very optimistic. I personally do not want to sit on these boxes, even though I’ve bought a few. 
I think these will see very little return, if any, in the next 2-3 years. With the amount you are willing to put into these boxes, I would put them into a WoTC-era product. Those are both popular and scarce, and will definitely keep growing. As the first and second-generation Pokémon fans age, they will have deeper pockets and they will want the originals. These new S&M are “popular” but definitely not scarce, and its target generation is mostly still in middle school or high school.
I think some might rise eventually. It’s super hard to tell…
The thing that decides if you should buy is the ratio of real sales to speculation sales and amount of sales. What I mean is imagine the following:
A) sales 90% of norm, 90% real sales
B) sales 100% of norm, 80% real sales
C) sales 130% of norm, 60% real sales
in scenario C there will always be a glut on the market, with the possibility of net value decrease.
in scenario B is the norm and its value could be affected by cards in the set.
in scenario A has the greatest potential for price growth, but an undesirable set will see less price growth than a desirable set.
Note: what I call real sales is sales in which the boxes and packs are opened for the cards.
As many have said these sets are oversaturated. If you do decide to buy a box/boxes of current day sets, wait until the set is at least 2 years old. Many probably bought evolutions on release to keep sealed, and now a year later the set has been opened to oblivion and besides M charizard the singles are nearly worthless. That said I think evolutions will be ok for reasons oldskoolpokemon mentioned, but evolutions (and all new set) will take at least 3 years (more likely 5+ if pokemon company is going to just roaring skies/primal clash/ancient origins the sets) to see growth (and I don’t think any set from here on out will go near flashfire)
What makes flash fire special compared to all the others?
Less people realized if you sat on a box a long time it would grow, and flashfire has Charizard. Now everyone and their mom bets on charizard, but in return the product gets opened to oblivion (evolutions) and there’s probably a lot of people sitting on the boxes as well. I suspect those who think it or any future charizard set will become the next flashfire will be dissapointed. Those who want to sit on any new set might be dissapointed unless they are cool waiting for it to be 5+ years old.