Frankenstein: Are All Classics Worth Reading?

I’m working on writing a Gothic novel for young adults, and as part of my process, I’ve been immersing myself in classic and contemporary Gothic literature.  One of the classics I finally picked up was Frankenstein.   I expected the dark gloomy atmosphere of the movies, the lightning clashes and dark mansions
There was some of this atmosphere and Shelley included beautiful descriptions including things like:

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A grin was on the face of the monster; he seemed to jeer, as with his fiendish finger he pointed towards the corpse of my wife. I rushed towards the window, and drawing a pistol from my bosom, fired; but he eluded me, leaped from his station, and running with the swiftness of lightning, plunged into the lake.

I loved this description, and don’t even get me started on the tragedy of Justine, who is often left out (or severely distorted and mangled) in most adaptations. 
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But there are also long passages of dialogue.  If you ever fell in love with the old radio or film adaptations with an almost insensible green-faced monster stumbling around mumbling…

Your perceptions are not based on the original because Frankenstein’s monster is learned and eloquent:

Be calm! I entreat you to hear me before you give vent to your hatred on my devoted head. Have I not suffered enough, that you seek to increase my misery? Life, although it may only be an accumulation of anguish, is dear to me, and I will defend it. Remember, thou hast made me more powerful than thyself; my height is superior to thine, my joints more supple. But I will not be tempted to set myself in opposition to thee. I am thy creature, and I will be even mild and docile to my natural lord and king if thou wilt also perform thy part, the which thou owest me. Oh, Frankenstein, be not equitable to every other and trample upon me alone, to whom thy justice, and even thy clemency and affection, is most due. Remember that I am thy creature; I ought to be thy Adam, but I am rather the fallen angel, whom thou drivest from joy for no misdeed. Everywhere I see bliss, from which I alone am irrevocably excluded. I was benevolent and good; misery made me a fiend. Make me happy, and I shall again be virtuous.

These speeches slow the narrative and preach the author’s themes in that “telling” instead of “showing” technique authors today are always warned to avoid. 

​Would there be a better way to show the creature to be the “fallen angel” he claims here?    And how did he learn to speak with such eloquence?

After reading the original, I moved on to read several contemporary adaptations, and I must admit I enjoyed them more in terms of pure entertainment.  This isn’t to say there’s no value in Shelley’s work.  It is a classic for a reason, and there are elements of her story that  but I wonder if one of the reasons it’s been adapted so often is that it simply does not stand the test of time.  As modern readers used to horrors like those found in Edgar Allan Poe, Ambrose Bierce, HP Lovecraft, Stephen King, and so many others, Shelley’s slow-moving novel may seem tame and didactic by comparison.  So we add an inscrutable monster who cannot speak, or even, in the most faithful adaptation by Kenneth Braunaugh, there is a second monster added for dramatic effect.
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I finished the book the same way I did The Blithedale Romance in graduate school—because I felt like I had to.  Perhaps my life, like many others, is too fast-paced to enjoy some of the slower moving classics.  Or perhaps my taste in literature has become horribly lowbrow.  In any case, I shall continue to enjoy some of the adaptations and leave the original alone.