'Writing' as a career is usually multiple career streams involving different types of writing skills in different markets.
These are some of the basic writing career streams:
Modern writers do have quite a few advantages in terms of options for preferred types of work. The Internet has made writing a global career market, and many writers do in fact work around the world from home. Most commercial writing is contract-based, and may also include outsourcing jobs from contractors like advertising agencies, web design firms, and marketing agencies.
Many writing jobs do require professional qualifications, but it's a matter of opinion whether or not those qualifications are absolutely essential. Large numbers of writers do not have qualifications, but they do have a lot of experience in commercial writing. If you check out the job ads for additional writers, you'll see quite a mixture of requirements.
Pay close attention to the requirements in your preferred field of writing. For technical writers, you'll note that qualifications in the science or professional field are the primary qualifications. For commercial writing, you'll see that experience is one of the primary requirements.
In practice, agencies outsourcing writing work are more interested in quality than qualifications. If you're a good writer with a good knowledge of your subjects and experience in the areas required, you can get any job, anywhere in the world.
Writing is a true 'portfolio career'. You're as good as your published work, which needs to be of a very high standard to get regular paying jobs in the commercial field. You also need to be highly productive, with a good, trustworthy production routine and the ability to meet and beat deadlines.
Standards of work do matter. In the commercial field, your work should be in 'ready to publish' condition before you send it to your clients. There are two good reasons for this:
Career progression for writers is based on creating both a good market for their writing and creating good, reliable income streams. Many writers actually operate multiple writing contracts, which not only provides good cash flow, but acts as an insurance against dependency on individual contracts.
The modern writing market does present huge opportunities, if you know how to take advantage of them. The writing market wants good quality content, and that really is the bottom line. If your quality is good, you'll find that you can get more work than you can handle.
Writing careers really can be a great commercial success, and a lot of fun. If you have the right combination of talent and commercial business sense, you can have a terrific career.