1) Replicas - this is not a problem in my opinion. If there will be many fake minifigs, even sets, buyers will be more careful and will avoid purchases from risky regions, or BL shops without good reputation. Serious BL shops will supply from serious sources (toy shops, serious web shops, etc), so there would be just a minimum fake stuff. The most problem would be minifigures, but this just move the buyers from minifigures to buying the MISB sets.
2) 3D printing - The same as (1). Plus I have doubts that we can see products from 3D printers comparable with original stuff from TLG.
3) Economy - this is a global problem, which can affect every area of investing. The lego fans are collectors, and collector stuff is sometimes more stable during unstable periods.
4) TLG campaign against resellers - this is even more platonic threat than activities of publishing companies against the music piracy. If TLG ban some resellers, they can easy register themselves through their friends, family members, etc. This is much more easy in countries, when are brand lego stores, where you can buy as much sets as possible. More effective from TLG would be a) extending the life of sets, faking with EOL, c) reedition of popular sets. But TLG is limited with these instrument - they cannot extend the life of sets extremly, since they need to change their portfolio (they cannot produce 2-3 time more sets than usual, plus its hard to sell for example 10 modulars in the same time due the risk of canibalism effect). We can also react for such activity and buy these sets later, for example after 5-6 years from the release date. But of course, such things will decline our profits - because if a certai set is selling on primary market long time, more AFOLs would have a chance to buy it.
5) a grow of investors - this is definitely the most important factor. If the number of investors (and number of sets stored in their cellars) reach a critical level, this can attack the ratio of supply and demand. Because if many investors will clean up most of exclusive sets in a certain country, what TLG will do? They will see that there is a big demand for certain sets, and they will produce more and more sets (which normally wouldnt be produced). This can lead to a big deformation of the market. The critical time can be a year after EOL - there will be huge stocks in investors hands, but the demand should be very limited - since most of collectors bought such sets in normal way before EOL. The situation can be better after longer time, when more new AFOLs will come. The question is whether we can expect more new collectors than the number of sets in the secondary market. I dont know. I am still an optimist - because if I buy a set, which normal price is 100 usd, for discounted price 80 usd, even If I sell this set for standard price, I have a profit 25%. So I need just a small growth of the price, or be more patient and wait for a longer time. I think that the profit will be smaller and smaller, but still there will be some profit - for example 15-25% per year. The investors just need to be more careful and choose just the best sets as a investors targets. For sure there will be a solid demand for perfect lego models, such as Unimog. I cannot imagine that I can have a problem to sell such set, bought for 150 usd, for 200-250 usd. Or a big fan of hobits will become a AFOL and find, that X years ago there were lego sets with favourite characters - I cannot imagine that such fan will refuse to pay 200 usd for Helms deep - he must know that such sets will never be produced again. Certain sets are unrepeatable. We just need to be very careful and dont waste our time, money and storage place for crap.