We get this type of question very often here. So let's get a few things straight and very clear:
1. There is an extremely low probability for someone to work in physics or as a physicist with just a B.Sc degree. Even with a M.Sc degree, it will be a struggle especially considering that there are people with PhDs seeking the same type of jobs. Now, it doesn't mean that you are not employable in other areas. But within physics itself, it will be difficult, especially if you want to do research work, without a PhD.
2. The degree of employability depends strongly on the area of physics you specialize in, and what skills and knowledge that you posses. I've posted, in another thread, an area of physics (accelerator physics) in which a lot of graduates have a high probability of getting employed simply because of the nature of the field and what these graduates can do. Read it! And yet, at the same time, we have high-energy physics theorists who can't find jobs, even outside of their fields. So asking about the possibility of employment with a physics PhD is as meaningful as asking for the possibility of employment with
A PhD degree! Things can vary wildly from one field/expertise to another!
3. Figure out what you like. Then figure out the employment picture in that area that you like. Now weigh the two against each other. What is your comfort level in taking such a risk? Some people have a higher tolerance for taking such a risk than others. We all have different backgrounds, different aspirations, different economic origins, etc. Each one of these factors can dictate how much we are willing to go to pursue what we want to do, and when it isn't worth the risk. No one else but you can decide that. However, you need to know both sides of the equation, which is what you wish to pursue, and what are the odds of working in that field. Only with clear and sufficient information should you make such a decision. And it may even change over time as you not only learn more about it, but also as times and circumstances change.
If anyone has looked at the series of advices that I've dished out on here, you would have seen me give encouragement to someone who seemedto be skeptical of pursuing a career in physics, and you would have also seen me try to inject some reality of employment (or lack of it) to someone who was dead-set on pursing a specific field in physics. I mean, look at this thread, for instance:
https://www.physicsforums.com/showthread.php?t=724673
And then look at the poll that I did on here:
https://www.physicsforums.com/showthread.php?t=667559
So on one hand, I'm all for students getting into physics, and on the other hand, I'm painting these vivid, sometime discouraging, picture of the struggle for employment in the every same field. Am I being inconsistent and contradictory? Nope!
And this is because of what I said earlier. You need to know BOTH factors before you decide. If you know fully well that the chances of getting employment in string theory and getting a faculty position is very low, and you still decide to pursue majoring in string theory, then all power to you! You are choosing something with your eyes wide open. And if you can't find a job in that field, there shouldn't be any sense of resentment or bitterness, because you knew fully-well of your chances and the risk that you are taking.
What we can do here is simply to give you the avenue to do what you wish to pursue, and also provide you with the information about the nature of the employment pattern in such-and-such an area. You'll hear many anecdotal accounts, often conflicting ones, that you will have to evaluate for yourself. You will also need to do your own research, look at many statistics, and figure out on your own at what the scenario is. The more you know, the better informed you are at making your decision. This point should not be a surprise.
Zz.