There are really two questions here: 'will this make them worse at swimming?' and 'will this be enough of a problem to kill them all off?'
The answer to the second one is easy: no, it won't.
Humans aren't exactly helpless in the water, after all, and your merfolk are both quite a lot smarter and (presumably) better swimmers than we are. Working together would make a huge difference, as would a few simple inventions: fin, spear, fishing sling.
If they're smart enough and technologically advanced enough there's no end to the fancy tricks that are available, but they don't need anything like that at all - a long pointy stick does a lot to level the playing field all on its own.
The first question is where things get interesting.
Having a tail made up of two parts instead of one isn't necessarily going to slow them down at all, but there are a few places where there are potential costs to the arrangement (and at least one potential benefit).
If they don't have flukes or something similar, they'll lose quite a lot of speed and control. This is the major issue. Whether it's their feet, flaps of skin on the ankles, or something else, they need to be able to duplicate the shape of a whale or dolphin's tail. (Or a seal's hind legs.) That broad, flat area plays a very important role in both propelling them forward and in letting them manoeuvre - it's effectively a combination propeller and rudder. If they don't have it, they're at a major disadvantage.
If water can pass between their legs as they're swimming, they lose power. If there's a gap where water can pass between their legs, they've effectively got two smaller 'tails' instead of one big one - and two half-sized tails don't add up to the same power as one full-sized one would have. If they can stop the water passing between their legs by pressing them together they get the power back, but it will mean they have to use extra energy while swimming and might tire more quickly. A monofin helps, as does wrapping the legs together.
If they have long leg bones instead of a spinal cord in the 'tail', they lose some effectiveness. A whale's tail can flex at any point along the length, and can bend in any direction. If you replace that with a single knee joint that only bends in one direction it costs you a tiny bit of efficiency for swimming forward, and a bit more for manoeuvring. (On the other hand, a rigid leg bone might be easier to brace to make sharp turns at top speed)
Being able to move their legs independently could make them more manoeuvrable. Being able to separate their legs gives them two independent control surfaces, instead of just one. While a single larger surface is more effective for simple turns (as noted above), it's not implausible that there are some complex manoeuvres that two legs can perform but a single tail couldn't.