It probably will not bring only a single cable type. It may make it so that higher-end cables are all Thunderbolt-capable, so that will help remove one difference that is present now. However, cost, physics, and market inertia aren't going to go away with the merging of the standards.
Cost: it is going to be cheaper to make a "power only" cable. The USB ecosystem has always maximized the number of vendors and competition to reduce costs. It will be cheaper to make a cable that just does power and USB 2.0, because it is just less copper, shielding, qualification, etc., which will all lead to a cheaper cable.
Similarly, we're going to have cables that are only Type-C on one side. Those also will toss anything that isn't needed to shave costs - Type-C to DisplayPort or Type-C to Type-A (the now-legacy USB previous generations).
Physics: There are still probably going to be 'active' versus 'passive' cables. If the USB4 standard makes it so USB can work with active cables, then the Thunderbolt distinction would disappear. It would become "shorter is passive and longer is active." It is unlikely, though, that USB will mandate that all new cables meet the higher throughput standard. That may require changing what the active cable standard is, so the current Thunderbolt v3 active cables may not become USB4 cables magically overnight.
With USB4 <--> USB 3.1 situations, the current Thunderbolt v3 cables probably won't identify with the USB 3.1 side. The "clean up" would more with USB4 <---> USB4 contexts, where the controllers are "aware" of the merged update on both sides.
It also won't be surprising if optical cables come back into the mix after the merger. There is a current USB 3.1 gen 2 optical cable being introduced. The longer the cable, the less power is probably going to get delivered on the other side, even sticking some copper into the mix.
Inertia: The humongous number of Type-A USB devices and even the smaller, but still relatively large, set of USB 3.1 [Type C] devices will still make cable distinction an issue. USB4 will probably benefit the "Thunderbolt" subset more as the number of devices will probably go up (e.g., custom controllers just for external drives that can flip between USB and Thunderbolt v3 and only talk to a SATA device that's just easier and cheaper to implement a drive with). USB4 drops the Type-A socket. Some products are going to highly resist moving.