Spec scripts don't sell like they used to.
What's a spec script? Basically, it's a script that's sold, not requested. In other words, if Marvel wants to make a movie, they'll decide on the movie and then hire Joe Screenwriter, their trusty go-to writer, to write it. But a spec script is when Jane Screenwriter writes a script for free on her own and then goes out to try and sell it.
Sure, your script could be great, and yeah, you just might sell it. But that's not how the majority of screenwriters make their money. If you want to make money as a screenwriter, you'll probably be better off selling yourself as a screenwriter, instead of just selling your script.
Most money-earning writers (even very successful ones) make money on writing assignments. That is, they are hired to work on a project in development. Be an ongoing money-maker for an agent. Be an ongoing money-maker for a production company. Don't be their one-hit pony.
Of course, if you want to put yourself on a pedestal, and say you're only willing to write your own original scripts, and you won't work for anyone else, then go for it. I wouldn't do that, but you're welcome to put all your eggs in that one tiny basket if you want.
So how do you get a writing assignment? You still have to write that awesome script. In fact, write a few of them. But when you reach out to reps and production companies, sell yourself as a writer, not the writer of this one script. Say something like, "If you love my script like a man loves his dog, consider me for open writing assignments like a centipede considers new socks!"
Absolutely do not write that in a query letter. BUT, do keep the message: I am not just the writer of this script- I am a writer for any upcoming projects.
Write that amazing script, but use it as a sample of what you can do.