As long as there is a market for one, the other will be around since they both start off as the same stock.
Not really, no. 120 and (small size thin) sheet film can both be made off the same base, but 135 generally is different - it lacks the gelatin anti-warp back coating of the bigger formats, and has a thicker base with a light-piping prevention dye.
135 has potential future issues as the required anti-light-piping substrate has no other application. If film volumes get too small to absorb the minimum amount that can be produced at a reasonable price, we might be out of a suitable acetate or polyester base, and the 120/sheet alternative, clear film, means that we'd have to load the cameras in a dark bag (which would predictably kill all applications outside the enthusiast market).
120 has rather complicated spooling and a backing paper with little to no other applications - issues with the backing paper (like chemical fogging of the film or poor opacity) already are occurring among the smaller makers, and the last automatic 127 spooling machine was scrapped years ago (all 127 available in the last years was hand spooled by one blind staff member at Fotokemika). But the clear stock needed is shared with many other industries, spooling machines are a task for a small machine workshop, and any printer can provide the kind of mediocre backing paper the smaller makers use by the thousand, so the outside dependencies are resolvable even at a very small volume.
Film industry insiders tend to be more worried about the fate of 135, as it is dependent on one externally procured unique product that has a very high minimum production volume, and as its customer base is used to extremely competitive or even destructive pricing and probably would not stay along over a dramatic price increase or loss of daylight loading convenience.