Doing a Masters degree can be an extremely rewarding experience, and it does have many benefits. You gain new skills, get the opportunity to really develop an area that you're (hopefully) passionate about, and gain a qualification that can help you to stand out in a crowded job market. That said, a Masters is certainly not an easy route to take - as you've already identified, it is time-consuming and it can be very stressful as well as expensive. Completing a Masters course in any discipline requires dedication and passion.
It's very easy to dismiss courses in subjects that don't appear to directly relate to a job. However you'll probably find that a good MA course in Screenwriting and Film will not only develop your skills in writing but also in pitching and marketing your work. The course may also introduce you to to industry contacts, give you a better idea of the range of job opportunities available to you, and allow for you to undertake internships/placements within the industry. No degree course can guarantee you a job but a well-designed and reputable course should equip you with the skills to navigate a challenging job market, and help you to make the most of your passions and interests.
I returned to university to complete my MA in English Literature after 11 years in the workplace. I'm now doing my PhD and, for me, my Masters was definitely one of the best things I've ever done. It allowed me to develop as a researcher and become more confident in both defining and presenting my ideas. It also gave me a valuable insight into what life as a working academic is like, and helped me to confirm that I did want to pursue an academic career. I can't guarantee that my MA or my PhD will lead to an academic job - the job market is, as you say, extremely competitive. But I hope that the skills I have developed during my studies will stand me in good stead, and that my increased awareness of that job market will allow me to be more competitive within it. Plus, as
@r.uh says, there's a lot to be said for doing something you love - my PhD stipend is less than half of what I was earning in my previous job but it's enough to pay the bills and, whilst I miss the little luxuries I used to be able to treat myself to, I wouldn't go back for anything.
Ultimately only you can decide whether a Masters course is right for you. But if you're passionate about screenwriting and film, then you're almost certain to find the study emotionally rewarding, even if the financial rewards cannot be guaranteed.
Hope that helps!
Amy