Is Mystic Arcanum worth it?

Title. I can mention a specific build if needed, but essentially most of my Warlock builds are Fighter 1/ Warlock X. Around Warlock 11/12 I feel like my builds are mostly complete. From there, I see so many multiclass options that can compliment the build nicely (Fighter, Sorcerer, Rogue, Bard, etc.), but then I look at MA 7-9 and I'm thinking "do I REALLY want to give up those higher level spells?" But then the other voice in my head is like "well you only get one casting per LR, where you could get higher DPR/ more spell slots/ more useful abilities if you multiclass."

What are your thoughts?