I think it was a lot easier to start with the basics not too long ago because that is all there really was. When I was learning, I was quite proud of my atrocious looking website. I think starting from nothing but classic CSS these days does not have the impact that it once did, especially with all the well-designed websites out there. Bootstrap gets you a lot of easy-wins in terms of looks and ease of use, so I can see why beginners will reach for it.
I don't think you can force them to learn it, but you can continue being a good mentor to beginners. I am a firm believer that having good fundamentals transcends the hot new framework. CSS will always be CSS, whether in its basic form, in Bootstrap, or CSS in JS. I think there will come a time when a beginner is not able to solve a problem with Bootstrap and they might struggle. They might reach out for help or they may learn an aspect of CSS that they did not know before. It will just take time for them to gain more experience.
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