I just pulled in a class-based component into a modern React project. It just worked with no adjustment.
Now that's commitment to backward compatibility (and they can't have it any other way, they won't rewrite half of facebook just because their own framework changed).
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The major JS frameworks have all been around for 5 years; several for 10 (react turns 10 this May).
Compare that to the average tech employee tenure, or even the average lifetime of a startup company.
I just pulled in a class-based component into a modern React project. It just worked with no adjustment.
Now that's commitment to backward compatibility (and they can't have it any other way, they won't rewrite half of facebook just because their own framework changed).