Helium reached a scale orders of magnitude larger in 1/5th the time. As TTN is a community effort it can't really fail in a traditional sense, but it has been lapped several times over at this point.
I've not really dug into the details as to what solutions Helium has, but it is quite interesting to see how this experiment will play out.
A much more realistic risk is sudden insolvency of a specific vendor (who are also responsible for maintaining firmware). There are about 30 approved vendors, growing monthly, but some have a larger share of the mining pool than others. The community is anticipating long term business risks and devising mechanisms to prevent a sudden loss of a large percentage of nodes due to business risk.
I firmly believe there is a significant flywheel developing here and I recently left my FAANG job to build in this ecosystem.